Fixes yet another build failure (which however only affects the 0.22
branch).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/666

Fixes another build failure.

Fixes build failure in StickerCommands.cxx.

This is the correct thing to do for ports from which a signal ultimately
"originates", such as is the case with mpd.

release v0.21.16

Commit 13208bf5 added range support to
the `move` command, but applied the wrong offset to the `to` variable.
When the source range is before the current song, and the song thus
gets decremented by the range size, then the final destination offset
must also be decremented by the range size.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/663

This is the last missing piece for https://github.com/MusicPlayerDaemon/MPD/issues/662

Commit 29f78b18 continued.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/662

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

Without requesting the property, "good" WebDAV servers would not send
it, and so MPD could never recognize a directory, failing the database
update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

Add a runtime sanity check to avoid the assertion failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

release v0.21.15