Pull up following revision(s) (requested by nat in ticket #759):

sys/dev/audio.c: revision 1.367
sys/dev/audio.c: revision 1.369
sys/dev/audio.c: revision 1.376 (patch)
sys/dev/audio.c: revision 1.377 (patch)
sys/dev/audio.c: revision 1.378
sys/dev/audio.c: revision 1.382 (patch)
sys/dev/audio.c: revision 1.383 (patch)
sys/dev/audio.c: revision 1.384
sys/dev/audio.c: revision 1.388
sys/dev/audio.c: revision 1.389
sys/dev/audio.c: revision 1.395
sys/dev/audio.c: revision 1.396 (patch)
sys/dev/audio.c: revision 1.397
sys/dev/audio.c: revision 1.403
sys/dev/audio.c: revision 1.451
sys/dev/audio.c: revision 1.452
sys/dev/audiovar.h: revision 1.59 (patch)
sys/dev/aurateconv.c: revision 1.21 (patch)
sys/dev/auconv.c: revision 1.31 (patch)
sys/dev/audiovar.h: revision 1.60 (patch)

Broadcast all conditional variables if in being deactivated so no readers
or writers get stuck.

Fix a panic caused by opening pad(4)'s mixer before the corresponding
audio device has attached.

Addresses PR kern/52424.

Improve audio_set_vchan_defaults().
- Correct confused input/output parameters.
- Remove sc->{sc_channels, sc_precision, sc_frequency}.  They are
  the same as sc->sc_vchan_params.{channels, precision, sample_rate}.
The input parameter of audio_set_vchan_defaults() is now only
sc->sc_vchan_params.

Fix PR kern/52437

const-ify.

0 -> NULL in audioattach()

"bits" sounds better than "name" for argument name.
I feel expression (name / NBBY) a little strange.

The audio module will now compile with WARNS=5.

Typo in debug message.

Remove a duplicated line.

Add missing initialization of sc_rfilters in audioattach().

Remove dead codes.

sc->sc_opens never changes in this loop and as a result of
previous sc_audiochan cleanup "sc_opens == 0" is the same as
"sc_audiochan is empty".

Clean up audio_allocbufs().

As a result of sc_audiochan cleanup, it is easy to access sc_hwvc.

Clean up audio_open().

As a result of sc_audiochan cleanup, this loop is the same as last + 1.
hw_if->set_params is mandatory, so it will never be NULL.

CID-1427745: kill possible buffer overflows.

Revert my wrong r1.380 and add a comment instead.

(martin)