Another big chunk of code for multi-player support.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).

Replaces GLib's GError.

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.

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

Now that the player thread can handle SEEK commands while not (yet)
playing, we can remove the "pc.state" check from pc_seek().

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

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