Fixes deprecation warnings.

This fixes a deadlock bug introduced by 18076ac9.  After all, the
second Cond was necessary.

The problem: two threads can wait for a signal at the same time.  The
player thread waits for the output thread to finish playback.  The
main thread waits for the player thread to complete a command.  The
output thread finishes playback, and sends a signal, which
unfortunately does not wake up the player thread, but the main
thread.  The main thread sees that the command is still not finished,
and waits again.  The signal is lost forever, and MPD is deadlocked.

The main_cond variable was completely unnecessary.  The pc.cond object
can be used for both main->pc and pc->main.

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

Fixes build problems with mingw32.

This commit reimplements the core of the "single" mode.  Instead of
doing the detection in the playlist code from the outside, it is moved
to the player thread, which gets a new option called "border_pause".
It will now pause playback exactly at the beginning of the new song,
making the feature more reliable.

Now that the player thread knows what will happen, it can suppress
cross-fading.

Fixes mantis tickets 0003055 and 0003166.

Make sure the player "owns" the next_song object, so nobody else can
free it.

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

Just in case "song" becomes invalid at some point.

Replaces _current_song().

Rewrite of the pc_get_error_message() function, now using a GError
object instead of the complicated "errored_song" attribute.

Previously, the condition "defined(play_audio_format)" was used to see
if an output device has been opened, but if the device had failed on
startup, an assertion failure could occur.  This patch adds a separate
flag.

Common function that manages "player" attributes after
audio_output_all_open() has returned.

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

When playing a CUE track, the player thread waited for the decoder to
become ready, and then sent a SEEK command to the beginning of the CUE
track.  If that is near the start of the song file, and the track is
short enough, the decoder could have finished decoding already at that
point, and seeking fails.

This commit makes this initial seek more robust: instead of letting
the player thread deal with the difficult timings, let the decoder API
emulate a SEEK command, and return it to the decoder plugin, as soon
as the plugin finishes its initialization.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

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.

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.