auth/kerberos: Create common helper to get the verified PAC from GSSAPI

auth/kerberos: Create common helper to get the verified PAC from GSSAPI

This only works for Heimdal and MIT Krb5 1.8, other versions will get
an ACCESS_DEINED error.

We no longer manually verify any details of the PAC in Samba for
GSSAPI logins, as we never had the information to do it properly, and
it is better to have the GSSAPI library handle it.

Andrew Bartlett