Before: raise, kill and pthread_kill delivered a signal at once or failed
with EINVAL, nothing stayed pending, and pause, sigwait, sigwaitinfo and
sigtimedwait called NotImplemented_nid_no_patch.
Now: a process-wide pending set holds signals 1 to 128. A signal raised
while it is blocked stays pending and is delivered when _sigprocmask
unblocks it, taken by sigwait, sigwaitinfo or sigtimedwait, or delivered by
pause, which runs its handler on the calling thread and returns -1 with
EINTR. pthread_kill to another thread leaves the signal pending for
whichever thread waits for it and answers ESRCH for a handle that is not a
live guest thread. sigtimedwait follows FreeBSD's sigtimedwait(2): EAGAIN
on timeout, EINVAL for a tv_nsec outside 0 to 1000000000 while nothing is
pending, and an indefinite wait with a null timeout.
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