MFC r358629:

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

git-svn-id: svn://svn.freebsd.org/base/stable/9@358880 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f