Implement forwarding of leaf TGTs to selected realms.

Refactor and enhance TGT forwarding to allow forwarding of leaf
(destination) TGTs for selected destination realms.

Enhance kinit(1) to renew non-origin realm tickets

Document delegate-destination-tgt

Use the newly implemented _krb5_mk_1cred().

lib/krb5/addr_families.c

@@ -1481,6 +1481,8 @@ krb5_make_addrport (krb5_context context,
     size_t len = addr->address.length + 2 + 4 * 4;
     u_char *p;
 
+    /* XXX Make this assume port == 0 -> port is absent */
+
     *res = malloc (sizeof(**res));
     if (*res == NULL)
 	return krb5_enomem(context);