r6727: One more step down the long march to the 'Kerberos domain join'.

r6727: One more step down the long march to the 'Kerberos domain join'.

This patch allows a suitably patched Heimdal GSSAPI library (detected
in configure) to supply to us the session keys, and further compleats
the gensec_gssapi module.  This is tested for CIFS, but fails for LDAP
at this point (that is what I'll work on next).

We currently fill out the 'session info' from the SAM, like
gensec_krb5 does, but both will need to use the PAC extraction
functions in the near future.

Andrew Bartlett