gsskrb5: Check dst-TGT pokicy at store time

Our initiator supports configuration-driven delegation of destination
TGTs.

This commit adds acceptor-side handling of destination TGT policy to
reject storing of non-destination TGTs when destination TGTs are
desired.

Currently we use the same appdefault for this.

Background:

    A root TGT is one of the form krbtgt/REALM@SAME-REALM.

    A destination TGT is a root TGT for the same realm as the acceptor
    service's realm.

    Normally clients delegate a root TGT for the client's realm.

    In some deployments clients may want to delegate destination TGTs as
    a form of constrained delegation: so that the destination service
    cannot use the delegated credential to impersonate the client
    principal to services in its home realm (due to KDC lineage/transit
    checks).  In those deployments there may not even be a route back to
    the KDCs of the client's realm, and attempting to use a
    non-destination TGT might even lead to timeouts.

lib/gssapi/krb5/creds.c

@@ -268,6 +268,7 @@ _gsskrb5_import_cred(OM_uint32 * minor_status,
     }
 
     handle->usage = GSS_C_INITIATE;
+    handle->destination_realm = NULL;
     krb5_cc_get_principal(context, id, &handle->principal);
     handle->ccache = id;
     handle->cred_flags = flags;