r17837: Split out the storing of memory cached credentials

r17837: Split out the storing of memory cached credentials
from the krb5 ticket renewal code. This allows cached
credentials to be stored for single sign-on via ntlm_auth
for machines in a domain still using NTLM. Also (hopefully)
fixes the reference counting problem with pam_logon/logoff
so multiple logons/logoffs won't lose cached credentials.
This compiles, but I'm intending to test it over the weekend
so don't complain too much :-). I also want it in the tree
so Coverity can scan it for errors. Guenther, check this over
please - I ran through the architecture with Jerry and he's
ok with it, but this is modifying your code a lot.
Jeremy.
(This used to be commit 679eeeb91155dad3942efde6ae9f8d81faf18c5b)