MFC r292440:

proc: fix a race which could result in dereference of bad p_pgrp pointer on
fork

During fork p_starcopy - p_endcopy area of a process is populated with bcopy
with only proc lock held. Another forking thread can find such a process and
proceed to access p_pgrp included in said area.

Fix the problem by moving the field outside. It is being properly assigned
later.