MFC r358629:
Implement a detaching flag for the sound(4) subsystem to take
appropriate actions when we are trying to detach an audio device,
but cannot because someone is using it.

This avoids applications having to wait for the DSP read data
timeout before they receive any error indication.
Tested with virtual_oss(8).

Remove some unused definitions while at it.

PR: 194727
Sponsored by: Mellanox Technologies

MFC r282650 and r282651:
Extend the maximum number of allowed PCM channels in a PCM stream to
127 and decrease the maximum number of sub-channels to 1. These
definitions are only used inside the kernel and can be changed later
if more than one sub-channel is desired. This has been done to allow
so-called USB audio rack modules to work with FreeBSD.

Add support for more than 8 audio channels per PCM stream for USB
audio class compliant devices under FreeBSD. Tested using 16 recording
and 16 playback audio channels simultaneously.

Bump the FreeBSD version to force recompiling all external modules.

MFC r282650 and r282651:
Extend the maximum number of allowed PCM channels in a PCM stream to
127 and decrease the maximum number of sub-channels to 1. These
definitions are only used inside the kernel and can be changed later
if more than one sub-channel is desired. This has been done to allow
so-called USB audio rack modules to work with FreeBSD.

Add support for more than 8 audio channels per PCM stream for USB
audio class compliant devices under FreeBSD. Tested using 16 recording
and 16 playback audio channels simultaneously.

Bump the FreeBSD version to force recompiling all external modules.