If the environment variable $HOME does not exist, don't attempt to
obtain it from /etc/passwd; without $HOME, the calling process
indicates that it does not wish MPD to access the home directory.

This also prevents MPD from attempting to load
`/root/.config/mpd/mpd.conf` if MPD got started as global systemd
service.  Reading from there makes no sense, only /etc/mpd.conf shall
be used then.

This piece of code was initially added by commit 5d857921.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1687

Fixes spurious linker errors.

Eliminate global variables, convert them to MixerMemento fields.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1583

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1536

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1337

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1328

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1285

This commit also increases the PROTOCOL_VERSION so clients can detect
the availability of the feature.

This reverts commit 6ed4aff4.  The
Meson wrap is broken on non-x86, because it unconditionally includes
<cpuid.h> which is an x86 only header.

Requires Meson 0.56.0:
 https://mesonbuild.com/Release-notes-for-0-56-0.html#per-subproject-warning_level-option

Requires Meson 0.54.0:
 https://mesonbuild.com/Release-notes-for-0-54-0.html#per-subproject-default_library-and-werror-options

I want to use per-subproject warning_level (0.56.0), default_library
(0.54.0).