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fork: rework locking around do_fork

- move allproc lock into the func, it is of no use prior to it
- the code would lock p1 and p2 while holding allproc to partially
construct it after it gets added to the list. instead we can do the
work prior to adding anything.
- protect lastpid with procid_lock

As a side effect we do less work with allproc held.

Sponsored by:   The FreeBSD Foundation

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