Eliminate dependency on io_thread_get().

The old struct config_param remains only for top-level string options.

Return false on error, not nullptr.

Reduce number of libjack calls.

What JACK expects is already implemented in MPD, just not used.  The
sample format conversion code in the JACK plugin was redundant and
could reduce sound quality.

Replaces GLib's g_strsplit().

Make it "const".