Save Musicbrainz ID to local CDDB

[poddmo]

When writing to the local CDDB, if a mbid exists, write that value into CDDBID.mbid

eg

if [ -s "${ABCDETEMPDIR}/mbid.$(checkstatus cddb-choice)" ]; then
    cp "${ABCDETEMPDIR}/mbid.$(checkstatus cddb-choice)" $HOME/.cddb/2604e204.mbid
fi

resulting in a CDDB entry:

:~/Music/abcde.2604e204$ ls -l $HOME/.cddb/2604e204*
-rw-rw-r-- 1 user group 551 Jan 24 23:20 /home/josh/.cddb/2604e204
-rw-rw-r-- 1 user group   36 Jan 24 23:27 /home/josh/.cddb/2604e204.mbid

Read the value back when using the locally cached CDDB. This will make future Musicbrainz cover art searches possible

[poddmo]

Also save the MusicBrainz Disc ID as a flac tag:
MUSICBRAINZ_DISCID: Identifies the disc using a calculated disc ID from the CD's track structure.