When process is exiting, check for suspension regardless of
multithreaded status of the process.

The stopped state must be cleared before P_WEXIT is set. A stop
signal delivered just before first PROC_LOCK() block in exit1(9) would
put the process into pending stop with P_WEXIT set or assertion
triggered. Also recheck for the suspension after failed
thread_single(9) call, since process lock could be dropped.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week