This reverts commit 552c30ea.

It has caused various problems; for example, MPD wasn't able to write
the pid_file (which was already mitigated by commit a4e42172).

And apparently, the socket file created in the same directory by
mpd.socket disappears when mpd.service (re)creates the directory.  I
could not reproduce this problem with 247.3, but maybe this is a bug
in older systemd versions?

Until we figure out why this happens, let's remove the
RuntimeDirectory directive.  A future MPD version may be launched as
regular user, not as root, which will eliminate one major problem with
RuntimeDirectory.

This way, MPD can reliably detect whether it was started as systemd
service, which is better than checking sd_booted(), which only checks
whether systemd manages all services, but still MPD could be started
manually.

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!