Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

The decoder_plugin structs must never change.  Don't work with
non-const pointers.

Nearly all mapper functions can fail and will then return NULL.  Add
checks to all callers.

Don't load non-regular files.

Clients which have authenticated via unix socket may add local files
to the MPD playlist, provided that they own the file.

The mapper library maps directory and song objects to file system
paths.  With this central library, the code mixture in path.c should
be cleaned up, and we will be able to add neat features like aliasing.

song_get_url() doesn't modify the song object.

isMusic() used to be a very inefficient function: with every
invocation, it did another stat() on the specified file.  There is
only one caller, do the stat() there manually and use hasMusicSuffix()
instead of isMusic().

CamelCase is ugly, rename the functions.

The runtime check suggests that the author has somehow thought
song_get_url(NULL) might be valid.  It should not be.  Replace it with
an assertion.

Eliminate duplicated code.

Instead of returning 0 or -1, return true on success and false on
failure.  This seems more natural, and when the C library was
designed, there was no "bool" data type.

This function was never used on remote songs.  Replace the runtime
check with an assertion.

CamelCase is ugly...  rename all functions.

Provide separate constructors for creating a remote song, a local
song, and one for loading data from a song file.  This way, we can add
more assertions.

When there are standardized headers, use these instead of the bloated
os_compat.h.

Again, a data type which can be forward-declared.

The struct can be forward-declared by other headers, which relaxes the
header dependencies.

This makes the update code thread-safe and doesn't penalize
the playlist code by complicating it with complicated and
error-prone locks (and the associated overhead, not everybody
has a thread-implementation as good as NPTL).

The update task blocks during the delete; but the update task is
a slow task anyways so we can block w/o people caring too much.

This was also our only freeSong call site, so remove that
function.

Note that deleting entire directories is not fully thread-safe,
yet; as their traversals are not yet locked.

song objects cannot exist without a path or URL

We already know if a song is a URL or not based on whether it
has parentDir defined or not.  Hopefully one day in the future
we can drop HTTP support from MPD entirely when an HTTP
filesystem comes along and we can access streams via open(2).

The "packed" attribute may have negative side effects on performance.
Remove the "packed" attribute, and increase the size of "song.url" to
a multiple of the machine word size.

Reduce the number of allocations we make, so there's less
pressure on the allocator and less overhead to keep track
of the allocations in.

It didn't save us any lines of code nor did it do anything
useful since we would overwrite everything anyways.

There's no reason to scan the playlist for a song we
just allocated.

Why have a "_func" prefix on all method names?  Also don't typedef the
methods, there is no advantage in that.

Having an enum type is much nicer than an anonymous integer plus CPP
macros.  Note that the old code didn't save any space by declaring the
variable 8 bit, due to padding.

SongList has been superseded by struct songvec.

Our linked-list implementation is wasteful and the
SongList isn't modified enough to benefit from being a linked
list.  So use a more compact array of song pointers which
saves ~200K on a library with ~9K songs (on x86-32).

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.

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.

If many tag_items are added at once while the tag cache is being
loaded, manage these items in a static fixed list, instead of
reallocating the list with every newly created item.  This reduces
heap fragmentation.

Massif results again:

 mk before:  total 12,837,632; useful 10,626,383; extra 2,211,249
 mk now:     total 12,736,720; useful 10,626,383; extra 2,110,337

The "useful" value is the same since this patch only changes the way
we allocate the same amount of memory, but heap fragmentation was
reduced by 5%.

The function newNullSong() is only used internally in song.c.

Since inputPlugin.c manages the list of registered decoders, we should
rename the source file.