[Development report #24] Audio Stack Improvements

MFC’d previous weeks’ patches to stable/14.

snd_dummy: Fix callout(9) races:
https://reviews.freebsd.org/D47459

	I am investigating one more callout(9) use-after-free panic I
	sometimes stumble upon with KASAN enabled, even with the patch
	applied.

sound: Fix chn_trigger() and vchan_trigger() races:
https://reviews.freebsd.org/D47461

sound: Fix hot-unload panics:
https://reviews.freebsd.org/D47462

sound: Remove PCM_DETACHING(), SD_F_DETACHING and SD_F_DYING:
https://reviews.freebsd.org/D47463

sound: Add initial sanity check in pcm_unregister():
https://reviews.freebsd.org/D47665

sound: Remove CHN_F_SLEEPING:
https://reviews.freebsd.org/D47559

	Fixes chn_sleep() panic reported by dougm@ and pho@ using
	stress2.

sound: Remove unused CHN_F_SILENCE:
https://reviews.freebsd.org/D47664

sound: Implement AFMT_FLOAT support:
https://reviews.freebsd.org/D47638

	Also notified bug reports to test it and received very positive
	feedback so far, since it fixes quite a few ports, including
	emulators/wine.

beep(1): Use AFMT_FLOAT:
https://reviews.freebsd.org/D47639

Document __FreeBSD_version 1500027:
https://cgit.freebsd.org/doc/commit/?id=3b840c7732f348af1052365fa5abd9ca99ef057e

Reviewed
https://cgit.freebsd.org/src/commit/?id=cdd3234186b19e1b9d7cfc985ef8d247552b0f11

Worked a bit on on-demand vchan allocation, but halted progress to work
on other things.

This work is sponsored by the FreeBSD Foundation.

Christos