Don't change realm in find_alternate_tgs

If a client makes a TGS request for a cross-realm TGS within a
different realm from the one we normally serve (e.g. the KDC realm is
X, and a client makes a TGS request for the server krbtgt/Y@Z), look
for alternate TGS principals within the requested server realm, not
the realm we normally serve.

This change shouldn't break any working well-formed TGS requests,
because changing the realm would trigger a failure in check_tgs_tgt.
It may fix some corner cases when multiple realms are served out of
the same KDB.  But primarily, this change makes referrals and aliases
easier to reason about, by eliminating a case where server->princ has
a different realm from request->server after the call to
search_sprinc().