Set the canonicalize flag in TGS requests and accept cross-realm referral tickets.

Set the canonicalize flag in TGS requests and accept cross-realm referral tickets.
We do not yet accept tickets in which the server name changes.

* krb5_sname_to_principal:   If there is no domain realm mapping return null realm
*krb5_get_cred_via_tkt: New behavior as described below

1) the referrals case:
  - check for TGT for initial realm
    - if a remote realm was specified (which must have happened via a
      domain_realm mapping), obtain a TGT for it the standard way and
      start with that.
  - use client realm for server if not specified
  - iterate through this loop:
    - request ticket with referrals turned on
    - if that fails:
      - if this was the first request, punt to non-referrals case
      - otherwise, retry once without referrals turned on then terminate
        either way
    - if it works, either use the service ticket or follow the referral path
    - if loop count exceeded, hardfail
2) the nonreferrals case
  - this is mostly the old walk_realm_tree TGT-finding (which allows
    limited shortcut referrals per 4120) followed by a standard tgs-req.
  - originally requested principal is used for this, although if we were
    handed something without a realm, determine a fallback realm based on
    DNS TXT records or a truncation of the domain name.

ticket: 2652
Owner: amb

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18598 dc483132-0cff-0310-8789-dd5450dbe970