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
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