Use new Jack Metadata API to mark CV ports.

By adding metadata key "http://jackaudio.org/metadata/signal-type" value "CV" pair to CV ports, patch bays can discriminate between audio and CV ports (SVN patchage e.g. can already do so).

Metadata API is only implemented in Jack1 for now, this patch will thus have no effect when linked to Jack1/2 without support for metadata.
Maybe silence -Wunused-result warnings on arch.