Closes https://github.com/MusicPlayerDaemon/MPD/issues/516

The two were never used at the same time, and merging them saves one allocation.

This transfers ownership of the buffer instead of adding another reference.

Fix another memory leak.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/514

A newly allocated buffer doesn't need this call; it only adds overhead
for copying the data.

av_frame_get_buffer() leaks memory if buffers were already allocated.

Fixes one of the memory leaks of https://github.com/MusicPlayerDaemon/MPD/issues/514

avio_read_partial() was added in libavformat 57.81.100, and we keep
compatibility with version 57.40 for now.  Fixes regression from
commit bfb7b011

Closes https://github.com/MusicPlayerDaemon/MPD/issues/511

Closes https://github.com/MusicPlayerDaemon/MPD/issues/510

.. and not "UNIX domain socket.  Be consistent about the naming.