Modify ip fw so that whenever UID or GID constraints exist in a

Modify ip fw so that whenever UID or GID constraints exist in a
ruleset, the pcb is looked up once per ipfw_chk() activation.

This is done by extracting the required information out of the PCB
and caching it to the ipfw_chk() stack. This should greatly reduce
PCB looking contention and speed up the processing of UID/GID based
firewall rules (especially with large UID/GID rulesets).

Some very basic benchmarks were taken which compares the number
of in_pcblookup_hash(9) activations to the number of firewall
rules containing UID/GID based contraints before and after this patch.

The results can be viewed here:
o http://people.freebsd.org/~csjp/ip_fw_pcb.png

Reviewed by:	andre, luigi, rwatson
Approved by:	bmilekic (mentor)