On exec, single-threading must be enforced before arguments space is

On exec, single-threading must be enforced before arguments space is
allocated from exec_map.  If many threads try to perform execve(2) in
parallel, the exec map is exhausted and some threads sleep
uninterruptible waiting for the map space.  Then, the thread which won
the race for the space allocation, cannot single-thread the process,
causing deadlock.

Reported and tested by:	pho (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks