Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Use LockSynchronousCommand() instead of SynchronousCommandLocked().
Fixes regression from commit ef663810 (dead lock due to cond_wait with
unlocked mutex).

Don't let the "client" pass its own GCond.  This was not used
consistently.

These are confusing, and since MixRamp development has ceased, not
useful to anybody.

Make sure the decoder "owns" the song object, so nobody else can free
it.

decoder_is_current_song() now recognizes different instances of the
same physical song.

Replaces _current_song().

Don't read song.start_ms and song.end_ms, let the player thread manage
this logic instead.

Remove the decoder dependency on player_control.  All player_control
was needed for is to signal the player thread, and we can do that with
a simple GCond as well.

Only dc_command_wait_locked() is really being used.

dc_new() allocates the object and returns it.  dc_free() frees it
(replaces dc_deinit()).

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

More abstraction for decoder_control.pipe.

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

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

Don't access decoder_control attributes directly.

These two variables are redundant, we need only one of them.

The START command returns without blocking; we don't need the
asynchronous decoder start anymore.

Don't wake up the target thread in every iteration of the wait() loop.
Waking it up once, right after the command has been set, must be
enough.

Allocate a decoder_control object where needed, and pass it around.
This will allow more than one decoder thread one day.

Use GMutex/GCond instead of the notify library.  Manually lock the
player_control object before accessing the protected attributes.  Use
the GCond object to notify the player thread and the main thread.

Replace decoder_control.notify with decoder_control.mutex and
decoder_control.cond.  Lock the mutex on all accesses to
decoder_control.command and decoder_control.state.