proc: implement pid hash locks and an iterator

forks, exits and waits are frequently stalled during poudriere -j 128 runs
due to killpg and process list exports performed for each package.

Both uses take the allproc lock. The latter case can be modified to iterate
over the hash with finer grained locking instead.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17817