Merge duplicate data.

When visiting a song, GetSong() was called, but this object was never
returned by calling ReturnSong().  This patch locks the database only
once in Visit() and passes the original song object to the visitor,
avoiding the copy.

Allow the function to be cancelled.

Fixes potential access to uninitialised memory.

This library crashes on out-of-memory (NULL pointer dereference).
There's not much useful MPD can do in such a situation, so let's
explicitly abort instead, just like GLib does.

Conflicts:
	src/queue_save.c

Implements Mantis ticket 0003549.

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.

With some linkers, it is important to specify the static library
first, and then libmpdclient.  Fixes linker failures.

Updates the WIN32 version number for the upcoming 0.18 release.

Auto-recover from soft errors.

Allow the plugin to allocate the GetSong() return value.