Make exit1() wait sysctl(2) `allprocess' loops.

Regardless on wired userland memory, KERN_FILE_BYPID and KERN_FILE_BYUID
`allprocess' loops have netlock provided sleep points, so concurrent
process exit(1) could crash kernel.

The main exit1() problem is that process teardown begins while process
is still linked to `allprocess' list, and current code doesn't allow to
unlink it first. Wait for concurrent sysctl(2) `allprocess' loops
between PS_EXITING bit setting and list unlinking. Both KERN_FILE_BYPID
and KERN_FILE_BYUID loops do PS_EXITING check and won't deal with dying
process. Concurrent exit1() thread will wait loops keeping process
linked to `allprocess' list.

Tested with i386 dpb(1) run.
Stress tests and ok bluhm.

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