The check was meant to fix an assertion failure, but it was the wrong
way around.  This broke cross-fading most of the time.

According to the mantis bug report 2847, there are several possible
variations of the "album artist" tag:

- "album artist"
- "album_artist"
- "albumartist"

This patch adds support for the latter two.

I've added PIPE_EVENT_SHUTDOWN because calling g_main_loop_quit() do not work when called from another thread.
Main thread was sleeping in g_poll() so I needed some way to wake it up.

By some strange reason call close(event_pipe[0]) in event_pipe_deinit() hangs.
In current implementation that code never reached so that was not a problem :-)
I've added a conditional to leave event_pipe[0] open on Win32.

An '\n' was erroneously inserted in the line containing the state, e.g.
"state: \nplay" instead of "state: play".

Fix for bug #2992.

htons() is not available if netinet/in.h is not included.

Fix gcc warning.

Work around aliasing warning.

Be consistent with the rest of MPD, and don't use the non-portable
header "malloc.h".

Fixes duplicate playlist entries.

False if the vector was not modified.

Send silence to all connected clients while paused, to avoid
connection interruption.

Avoid buffer underruns on the streaming client, if the encoder is "too
efficient" (e.g. when encoding silence while paused).

The ReplayGain filter clamped the gain to max. 100 % even if the
algorithm determined the signal needed a boost. That would result in any
such tracks being played with too low volume, effectively defeating the
purpose of the filter.

Unfortunately, there's no "optimized" implementation here.  We can't
use Linux's proprietary system call dup3(), because it would require
us to specify the new descriptor.

This also fixes installation when no *.gif files found in 'doc/api/html' folder.

This hack creates a copy of doc/doxygen.conf with the INPUT setting
replaced.

Free the temporary path string in apply_song_metadata().

Use the functions song_save() and song_load() to use the same format
as in the database file for those songs which need the tags.

While this is not useful for the database, it may become useful for
reusing this code for the state file.

Don't try db_get_song() if the URI has a scheme.

If a song with an absolute path points inside the music directory,
print only the relative part.  This happens when partial songs from a
playlist file were loaded.