Remove net lock from DIOC{S,G}ETLIMIT

Remove net lock from DIOC{S,G}ETLIMIT

Grab the pf lock for pf_pool_limits[] in pfsync such that all access is
covered by the pf lock;  document accordingly.

Hard memory pool limits don't need the net lock for protection, pool(9)s
have their own internal lock and the pf lock fully covers limit values.

(pf_pool_limits[] access in DIOCXCOMMIT remains under pf *and net* lock
 until the rest in there gets pulled out of the net lock.)

OK sashan