AIX51: Fix PAGs

AIX51: Fix PAGs

On AIX 5.1 and later, we set a process' PAG by using the AIX PAG
mechanism (and not by group ids), but we were determining what PAG a
process was in by the group list. Instead use the PAG identifier.

This effectively reverts 277c37f48c8126ba9cb986ffc7361fcb98e2bbf2, but
it puts the kcred_getpag call in a different place that makes more
sense in the current PAG code organization.

Change-Id: Ifbc2602cfa9f32bf5d9b53b71fc67fe0761394ea
Reviewed-on: http://gerrit.openafs.org/4479
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-on: http://gerrit.openafs.org/4503