alsa: Add OSS sequencer API emulation

alsa: Add OSS sequencer API emulation

Implement OSS /dev/sequencer and /dev/music emulation via the ALSA
sequencer API. Enumerate available MIDI ports at open time, subscribe
to them, and translate OSS 4/8-byte event records to/from ALSA
sequencer events.

A background pthread handles incoming ALSA events and converts them
to OSS format in a ring buffer; a pipe wakes poll/select waiters.
SNDCTL_SEQ_SYNC uses an ECHO event echo-back to synchronize with the
ALSA queue.

Signed-off-by: Takashi Iwai <tiwai@suse.de>