Make stop_all_proc_block interruptible to avoid deadlock with parallel suspension

If we try to single-thread a process which thread entered
procctl(REAP_KILL_SUBTREE), and sleeping waiting for us unlocking
stop_all_proc_blocker, we must be able to finish single-threading. This
requires the sleep to be interruptible.

Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D35310