[krbdev.mit.edu #7791] S4U2Self fails with Windows 2008

[Greg Hudson via RT]

I can reproduce this by breaking FAST recognition in the MIT KDC.  We also 
have a report that it fails with Heimdal, which is FAST-unaware.

When we encode the FAST TGS request, we move the S4U2Self padata into the 
FAST inner body.  A FAST-unaware KDC only sees the outer body and 
interprets the request as a regular TGS request, and issues a ticket for 
server -> server.  gc_via_tkt.c detects this at line 269 and bombs out 
with a locally generated KRB5KDC_ERR_PADATA_TYPE_NOSUPP.

I have asked Sam for advice on the best fix.