Catch up to dynamically-sized cr_groups in FBSD80

Catch up to dynamically-sized cr_groups in FBSD80

In FreeBSD 8.0 and later, (struct ucred)->cr_groups is a pointer
to a dynamically-allocated array, and NGROUPS is now 1024 by default
(tuneable at boot).
Don't put a gid_t[NGROUPS] on the stack for the FBSD80_ENV case.
Also avoid keeping a function-local ucred structure (in
afs_osi_proc2cred()), by bypassing that function entirely
(though this accesses the process credentials directly, which may
require locking; thread credentials accesses can be safely done
lock-free).  Add an osi_Panic() to ensure that it stays that way.
Don't pretend that we have a useful afs_osi_cred to export.
Don't blindly overwrite NGROUPS past cr_groups.

Change-Id: I76295164a24bddf2782ab2fa662acd0e1b4855d6