Fix various MIDI corruption bugs.

Fix various MIDI corruption bugs.
Re-enable MIDI CC controller bars and other immediate output (hans commented out, tsk tsk).
Write channel mode as textual enumeration instead of magic number.
Better atomic (almost) channel mode switching on MIDI ring buffer (was a possible, if unlikely, source of corruption).
Handle some cases of broken MIDI, and oversized events, more gracefully.

git-svn-id: svn://localhost/ardour2/branches/3.0@3335 d708f5d6-7413-0410-9779-e7cbd77b26cf