sigtimedwait: Use a unique wait channel for sleeping

[markj]

Summary

When a sigtimedwait(2) caller goes to sleep, it uses a wait channel of
p->p_sigacts with the proc lock as the interlock. However, p_sigacts
can be shared between processes if a child is created with
rfork(RFSIGSHARE | RFPROC). Thus we can end up with two threads
sleeping on the same wait channel using different locks, which is not
permitted.

Fix the problem simply by using a process-unique wait channel, following
the example of sigsuspend. I believe the actual wait channel value is
irrelevant here, sleeping threads are awoken using sleepq_abort().

Reported by: [e-mail]
Reported by: [e-mail]