Prepare for a ICU-based backend without GLib.

Don't expose pointer that requires the caller to invoke g_free(),
because that's GLib-only.

Don't abort the process, let the caller decide instead.

A priority should be a volatile thing: it should schedule a song for
playing back once, but it should not affect the next time the queue
gets played.

Empty currently, but code will be added song.

Fixes compiler warning.

This was used by proprietary software.  MPD adopted it a few years
ago, which turns out to be a mistake, because it now creates problems
for some MPD users (http://bugs.musicpd.org/view.php?id=4168).

Avoids crash because Close() invokes a call on a destructed
NfsConnection.