Refactor KDC TGS processing code

Refactor KDC TGS processing code

Split the TGS processing code into information gathering, constraint
and policy checking, and ticket-issuing steps, using a structure to
hold the gathered information.  Split validate_tgs_request() into
validate_tgs_constraints() and check_tgs_policy() for better auditing.
Fold kdc_process_s4u2proxy_req() into check_tgs_policy(), except for
the get_pac_princ_with_realm() step which is now performed in
gather_tgs_req_info().  Modify some other utility functions to fit the
new design.