Allow referrals for cross-realm S4U2Self requests

Allow referrals for cross-realm S4U2Self requests

According to MS-SFU 3.2.5.1.1, the KDC should issue a referral for
S4U2Self requests if the requesting service is not in the KDC's realm.
Commit 8a9909ff9ef6b51c5ed09ead6713888fbb34072f explicitly prevents
referrals for S4U2Self requests; on further analysis, this appears to
have been preserving a bug rather than applying a proper constraint.
However, we should not issue referrals for within-realm S4U2Self
requests.  (This should only come up if a server possesses a TGT but
its principal entry has been deleted.)

Remove the S4U2Self referral check in process_tgs_req().  Instead add
a more specific check in kdc_process_s4u2self_req(), adding new
parameters for the header server principal and a flag indicating
whether a referral is indicated.

[ghudson@mit.edu: rewrote commit message; adjusted style slightly]

ticket: 8747 (new)