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".

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

The MPD core will add this as a prefeix.

Replaces GLib's GError.