Remove uses of cv_waiters in PCM_RELEASE and CHN_BROADCAST, and also use
a counter around cv_timedwait_sig() in chn_sleep(), which is checked in
pcm_killchans(), as opposed to reading cv_waiters directly, which is a
layering violation.
While here, move CHN_BROADCAST below the channel lock operations.
Reported by: avg, jhb, markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 days