Add KDC support for X.509 S4U2Self requests

Add KDC support for X.509 S4U2Self requests

Add a KDB function krb5_db_get_s4u_x509_principal() and an associated
method in the DAL, bumping the minor version and cleaning up a
leftover comment in the table from major version 6.

When processing an AS-REQ, look up the client principal by certificate
if the request contains a non-empty PA-S4U-X509-USER value.  When
processing an S4U2Self TGS-REQ, allow requests with certificates, and
look up the client principal by certificate if one is presented.

[ghudson@mit.edu: factored out lookup_client() in AS code; rewrote
commit message and some comments; adjusted flow control changes in
kdc_process_s4u_x509_user()]

ticket: 8781 (new)