The tag pool is a shared global resource that is infrequently
modified.  However, it can occasionally be modified by several
threads, especially by the metadata_pipe for streaming metadata
(both reading/writing).

The bulk tag_item pool is NOT locked as currently only the
update thread uses it.

Trying to read or remember
  "tag->numOfItems * sizeof(*tag->items)"
requires too much thinking and mental effort on my part.

Also, favor "sizeof(struct mpd_tag)" over "sizeof(*tag->items)"
because the former is easier to read and follow, even though
the latter is easier to modify if the items member changes
to a different type.

The previous patch enabled these warnings.  In Eric's branch, they
were worked around with a generic deconst_ptr() function.  There are
several places where we can add "const" to pointers, and in others,
libraries want non-const strings.  In the latter, convert string
literals to "static char[]" variables - this takes the same space, and
seems safer than deconsting a string literal.

We're pretty careful about using const these days, so
enable these warnings to keep us that way.

All callers of fdprintf() have been converted to client_printf() or
fprintf(); it is time to remove this clumsy hack now.  We can also
remove client_print() which took a file descriptor as parameter.

Now that we have removed all invocations of client_get_fd(), we can
safely remove this transitional function.  All access to the file
descriptor is now hidden behind the interface declared in client.h.

Pass the client struct instead.

Pass the client struct instead of the raw file descriptor.

Pass the client struct instead of the raw file descriptor.

The shared code in showPlaylist() isn't worth it, because we aim to
remove fdprintf().  Duplicate this small function, and enable stdio
buffering for saved playlists.

Pass the client struct instead of the raw file descriptor.

Pass the client struct instead of the raw file descriptor.

String literals (including those defined in CPP macros) can be
concatenated at compile time.  This saves some CPU cycles in
vsnprintf() at run time.

commandError() has been superseded by command_error(), and is not
being used anymore.  Remove it.

The function loadPlaylist() wants to report incremental errors to the
client, for this reason we cannot remove its protocol dependency right
now.  Instead, make it use the client struct instead of the raw file
descriptor.

Don't pass the raw file descriptor around.  This migration patch is
rather large, because all of the sources have inter dependencies - we
have to change all of them at the same time.

Pass the client struct to CommandHandlerFunction and
CommandListHandlerFunction.  Most commands cannot take real advantage
of that yet, since most of them still work with the raw file
descriptor.

Instead of passing the file descriptor, pass the client struct to
getCommandEntryAndCheckArgcAndPermission().

These two functions take a client struct instead of the file
descriptor.  We will now begin passing the client struct around
instead of a raw file descriptor (which needed a linear lookup in the
client list to be useful).

No protocol code in the audio output library.

The "volume" library shouldn't talk to the client.  Move error
handling to command.c.

Again, move error handling to command.c.

Move another ocurrence of error handling over to command.c.

This patch continues the work of the previous patch: don't pass a file
descriptor at all to traverseAllIn().  Since this fd was only used to
report "directory not found" errors, we can easily move that check to
the caller.  This is a great relief, since it removes the dependency
on a client connection from a lot of enumeration functions.

Database traversal should be generic, and not bound to a client
connection.  This is the first step: no file descriptor for the
callback functions forEachSong() and forEachDir().  If a callback
needs the file descriptor, it has to be passed in the void*data
pointer somehow; some callbacks might need a new struct for passing
more than one parameter.  This might look a bit cumbersome right now,
but our goal is to have a clean API.

Continuing the effort of removing protocol specific calls from the
core libraries: let the command.c code call commandError() based on
PlaylistInfo's return value.

Return an "enum playlist_result" value instead of calling
commandError() in storedPlaylist.c.

The playlist library shouldn't talk to the client if possible.
Introduce the "enum playlist_result" type which the caller
(i.e. command.c) may use to generate an error message.

Make them both return void.

Client's input values should be validated by the command
implementation, and the core libraries shouldn't talk to the client
directly if possible.  Thus, setPlaylistRepeatStatus() and
setPlaylistRandomStatus() don't get the file descriptor, and cannot
fail (return void).

When an error occurs after the file has been opened, the function will
never close the FILE object.

The "fspath" argument of writeStoredPlaylistToPath() must never be
NULL.  There should be an assertion on that, instead of a run-time
check.

The function valid_playlist_name() checks the name, but it insists on
reporting an eventual error to the client.  The new function
is_valid_playlist_name() is more generic: it just returns a boolean,
and does not care what the caller will use it for.  The old function
valid_playlist_name() will be removed later.

Currently, when the tag cache is being serialized to hard disk, the
stdio buffer is flushed before every song, because tag_print.c
performs unbuffered writes on the raw file descriptor.  Unfortunately,
the fdprintf() API allows buffered I/O only for a client connection by
looking up the client pointer owning the file descriptor - for stdio,
this is not possible.  To re-enable proper stdio buffering, we have to
duplicate the tag_print.c code without fprintf() instead of our custom
fdprintf() hack.  Add this duplicated code to tag_save.c.

Move everything which dumps song information (via tag_print.c) to a
separate source file.  song_print.c gets code which writes song data
to the client; song_save.c is responsible for serializing songs from
the tag cache.

Move everything which dumps a tag to a file descriptor to tag_print.c.
This relaxes dependencies and splits the code into smaller parts.

Based on client_puts(), client_printf() is the successor of
fdprintf().  As soon as all fdprintf() callers have been rewritten to
use client_printf(), we can remove fdprintf().

client_write() writes a buffer to the client and buffers it if
required.  client_puts() does the same for a C string.  The next patch
will add more tools which will replace fdprintf() later.

As usual, include only headers which are really needed.

clearMpdTag could be called on a tag that was still in a
tag_begin_add transaction before tag_end_add is called.  This
was causing free() to attempt to operate on bulk.items; which is
un-free()-able.  Now instead we unmark the bulk.busy to avoid
committing the tags to the heap only to be immediately freed.

Additionally, we need to remember to call tag_end_add() when
a song is updated before we NULL song->tag to avoid tripping
an assertion the next time tag_begin_add() is called.