Setting `dns_lookup_realm = false` in `[libdefaults]` and setting name
canon rules that force the empty realm causes destination-TGT delegation
to break because the client doesn't know the service's realm.
Because MIT and Heimdal check that the (unauthenticated plaintext)
sname/realm of the Ticket in the KDC reply matches the sname/srealm in
the enc-part of the KDC reply, we know we can trust the realm of the
ticket found in the ccache. So use that.
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