Make GCPAGs_perproc_func cleaner for FBSD case

Make GCPAGs_perproc_func cleaner for FBSD case

Partially rewrite 2cf1f10a to reduce code clutter.
Instead of accessing pproc->p_ucred directly in afs_GCPAGs_perproc_func(),
push this access into a FBSD-specific afs_proc2cred() function.
This is not quite right, since we do not lock the proc mutex, but
in the same way as the old version and with more reasonable preprocessor
conditionals.
This also eliminates a probably-needless call to osi_Panic.

Change-Id: Ib403132f31cc13f8c4581cb89ecc8938d0a0dd88
Reviewed-on: http://gerrit.openafs.org/1672
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>