This enforces the "not nullptr" rule.

Use milliseconds precision to reduce rounding errors.

Enables using range-based "for".

Replaces GLib's GError.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Remove GLib dependency from some headers.

Instead of doing run-time initialisation, use a constant lookup table.

Clean up the process before exiting.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

The signal_check library went out of order when we started using the
GLib main loop.  Convert the SIGHUP handler to use event_pipe instead.

Most of these functions were obsoleted when we switched to threaded
MPD.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.