krb5_pac_verify and server key enctype extraction

[Sam Hartman]

I think we may want an interface to expose a verified PAC for 1.8.
Possibly something at least nominally compatible with the naming work
going on in kitten or that can be extended to that interface.  I'm
definitely not talking about name attributes for each pac subfield,
simply one attribute for the verified pac as a whole, which is not
present if the pac fails to verify.