If stickers are not configured at runtime, don't call
sticker_song_delete().

Export the "g_playlist" variable, and pass it to all playlist
functions.  This way, we can split playlist.c easier into separate
parts.  The code which initializes the singleton variable is moved to
playlist_global.c.

On some platforms, g_free() must be used for memory allocated by
GLib.  This patch intends to correct a lot of occurrences, but is
probably not complete.

When a song is deleted from the database, remove its sticker, too.

What's still missing is some sort of garbage collector after a fresh
database create (--create-db).

Without a music_directory, MPD is an excellent streaming client.

Don't use fixed static buffers.  GLib allocates a new string for us
anyway, let's just return this one instead of copying it.

The archive code was disabled, because config.h was not included and
thus ENABLE_ARCHIVE was not defined.

Save an empty database, even if the music directory is empty.

The notify library is easier to use, and has no disadvantages.

Handle the DELETE and UPDATE events in separate callbacks:
song_delete_event() safely deletes a song, and update_finished_event()
is called when database update is complete.

GLib mandates that you initialize all GError objects with NULL prior
to passing it.

Use GLib's g_build_filename() instead of pfx_dir().

directory_is_root() is cheaper than
isRootDirectory(directory_get_path()).

The struct delete_data has only one member left and can be eliminated.

Determine the suffix manually, and use decoder_plugin_from_suffix()
and archive_plugin_from_suffix() instead.

This way, song_file_update_inarchive() can be optimized: it does not
have to translate its path.

Reverse the condition: delete directories which don't exist anymore.
This typo caused a slowdown during partial database update.

WIN32 does have some kind of symbolic links (e.g. in NTFS), but the
readlink() function is not available.  Disable symlink checking for
now.

Don't use fixed stack buffers.

Don't use fixed stack buffers.

If we want to check whether a file is a directory, use GLib's
g_file_test() instead of stat().

Make the event_pipe (formerly main_notify) send/receive a set of
events, with a callback for each one.

The default event PIPE_EVENT_SIGNAL does not have a callback.  It
is still there for waking up the main thread, when it is waiting for
the player thread.

Continuing the previous patch.

We are going to migrate away from the concept of notifying the main
thread.  There should be events sent to it instead.  This patch starts
a series to implement that.

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

When there are no archive plugins, we do not need the archive API at
all.  Drop all its overhead.

MPD 0.13 and older followed all symbolic links.  Although this can be
a security problem (as it has always been), 0.14 should offer the same
default behaviour as 0.13.

The configuration options "follow_outside_symlinks" and
"follow_inside_symlinks" let the user control whether MPD should
follow symbolic links in the music directory.

[mk: converted variables to "bool"; moved configuration to
update_global_init()]

Those two functions are called when MPD starts and exits.  It allows
the update library to perform global initialization and
deinitialization.  The implementations are currently empty.