Add sig_intr(9).

It gives the answer would the thread sleep according to current state
of signals and suspensions. Of course the answer is racy and allows
for false-negatives (no sleep when signal is delivered after process
lock is dropped). Also the answer might change due to signal
rescheduling among threads in multi-threaded process.

Still it is the best approximation I can provide, to answering the
question was the thread interrupted.

Reviewed by: markj
Tested by: pho, rmacklem
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D26628