Fixes hide_playlist_targets not working after server restart

Currently, `hide_playlists_targets` works by skipping songs with
`in_playlist` value set to true in
[`Directory::Walk`](https://github.com/MusicPlayerDaemon/MPD/blob/a57bcd02382947abbd961594cdf00302c0c7866a/src/db/plugins/simple/Directory.cxx#L237). But
`in_playlist` is not stored and only updated in
[`UpdateWalk::PurgeDanglingFromPlaylists`](https://github.com/MusicPlayerDaemon/MPD/blob/a57bcd02382947abbd961594cdf00302c0c7866a/src/db/update/Playlist.cxx#L139),
which will only be executed while updating DB.

This causes the problem that playlist target songs are correctly
hidden after database update, but will remain visible after mpd server
restarted. This pr solves the problem by storing `in_playlist` value
of songs into the `SimpleDatabase` file.

This allows dropping a few compile-time version checks and we can use
pkg-config to detect the library.

This got lost 8 years ago in commit 87c88fcb

D'oh!

According to
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS_INIT.html
the _INIT variables should be set in the toolchain file.

Right now, zlib is only built for Windows, but we may eventually
changed that, so don't hard-code `win32/Makefile.gcc`.

Make sure all the gory details are visible in CI logs.

For better error logs on CI.

By default, if the parent of a process dies, the process gets SIGHUP
and is supposed to shut down.  This however doesn't work for MPD,
because MPD redefines SIGHUP with a different meaning (like most
daemons do).

To work around this, we configure the kernel to send SIGTERM instead
of SIGHUP.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1706

Fixes a busy loop in BufferingInputStream::RunThreadLocked() because
the method never learns that seeking is ignored, even though the HTTP
stream is already broken and can never be read; nobody cared to check
for errors.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1727

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