We use range-based for and nullptr, which was not implemented prior to
gcc 4.6.

Conflicts:
	src/locate.c

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

libvorbisidec and libvorbis export the same symbols, which is a
dangerous thing.  Since libvorbisenc depends on libvorbis, this can
get nasty, so let's disable the Vorbis encoder unless the user
explicitly wants it.

WavPack tags are always APEv2, by definition.  Reuse the tag_table
from tag_ape.c, instead of rolling our own.

Avoids clang pickiness.  This code is not correct, but we'll fix that
another day.

Using libopus and libogg.

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.