Remove some DNS_DISPATCHATTR flags

- DNS_DISPATCHATTR_CANREUSE was never set. the code that implements it
  has been removed.

- DNS_DISPATCHOPT_FIXEDID and DNS_DISPATCHATTR_FIXEDID were both
  defined, but only the DISPATCHOPT was ever set; it appears the
  DISPATCHATTR was added accidentally.

- DNS_DISPATCHATTR_NOLISTEN was set but never used.

lib/ns/interfacemgr.c

@@ -707,9 +707,6 @@ ns_interface_destroy(ns_interface_t *ifp) {
 
 	for (int disp = 0; disp < ifp->nudpdispatch; disp++) {
 		if (ifp->udpdispatch[disp] != NULL) {
-			dns_dispatch_changeattributes(
-				ifp->udpdispatch[disp], 0,
-				DNS_DISPATCHATTR_NOLISTEN);
 			dns_dispatch_detach(&(ifp->udpdispatch[disp]));
 		}
 	}