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.

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8, andd
apparently, nobody ever noticed.

This missing piece probably never really hurt, because
HttpdClient::OnSocketClosed() would be called right after a socket
error, but it's better to be explicit about closing on error.

Allows removing a CommandResult::CLOSE check.