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
Thread waiting for the vfork(2)-ed child to exec or exit, must allow
for the suspension.

Currently, the loop performs uninterruptible cv_wait(9) call, which
prevents suspension until child allows further execution of parent.
If child is stopped, suspension or single-threading is delayed
indefinitely.

Create a helper thread_suspend_check_needed() to identify the need for
a call to thread_suspend_check().  It is required since call to the
thread_suspend_check() cannot be safely done while owning the child
(p2) process lock.  Only when suspension is needed, drop p2 lock and
call thread_suspend_check().  Perform wait for cv with timeout, in
case suspend is requested after wait started; I do not see a better
way to interrupt the wait.

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