Fixes Travis failure with Meson 0.50:

 ERROR: Could not detect Ninja v1.5 or newer

Makes using the Java glue classes simpler to use, at the cost of very
little overhead.

Copy the system socket unit to the "user" directory.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/530

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

Closes https://github.com/MusicPlayerDaemon/MPD/issues/524

Closes https://github.com/MusicPlayerDaemon/MPD/issues/523

A first use case for our libchromaprint integration added by commit
30e22b75

If the DecoderThread gets woken up by a STOP command, this should be
detected as early as possible.

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.