rc_obsolete_vars is a list of pairs, so format it with one pair per line.
note midirecord.
- check sc->dying after sleeping in several more places, and
  convert it into EIO error where necessary.
- remove a wrong additional mutex_exit() call.
- make sure to check sc->dying under the device lock.
- fix a confusion between midi(4) unit and connected to sequencer
  devices.
- minor comment/debug clean ups.

fixes problems attempting to read or write from the right midi(4)
device using the sequencer(4) device when one or more of the
non-final devices fails to open with midiseq_open().
fix !AUDIO_DEBUG build.