MFC r338422: Adding support for CS46xx MIDI output.

With this patch, users can play the MIDI files through /dev/sequencer
device with tools like playmidi. The audio output will go through
the external MIDI device such like wavetable synthesis card.

Reviewed by: matk (a long time ago), kib
Tested with: Terratec SiXPack 5.1+ + Yamaha DB50XG

MFC r298339:
r298339 (by cem):

sound(4): Don't use-after-free in midi module unload

Also, use ANSI function parameter definitions (void) while here.

CID: 1006107

MFC r289888:
- Plugging a memory leak when malloc() failed during initialisation;
- Plugging another memory leak inside the destructor.

Reviewed by: matk

MFC r286887: Using the error return code documented in the comment.

Though there is no direct midi_uninit() caller amongst existing drivers
at this moment, a quick experiment indicates that EBUSY gives users more
precise error message once drivers start to honour this result. For example,
emu_midi_detach() should check the result of mpu401_uninit() and block
module unloading if there is any MIDI I/O in progress.

MFC r286886: Fixing typo as well as improving readability of a few comments.

MFC r286887: Using the error return code documented in the comment.

Though there is no direct midi_uninit() caller amongst existing drivers
at this moment, a quick experiment indicates that EBUSY gives users more
precise error message once drivers start to honour this result. For example,
emu_midi_detach() should check the result of mpu401_uninit() and block
module unloading if there is any MIDI I/O in progress.

MFC r286886: Fixing typo as well as improving readability of a few comments.