The io_uring buffer is allocated as memlocked memory, as MPD needs to
be able to lock memory.

On linux-rt, kernel IRQ threads are configured with priority=50, and
this change configures MPD somewhat below that priority, leaving some
room for other programs to be configured in between.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/643

This allows using the file as a user unit, where "%t" maps to
"$XDG_RUNTIME_DIR".

Proposed in https://github.com/MusicPlayerDaemon/MPD/issues/530

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!