Simplify dns_dispatchmgr_create with fixed buffersize

- UDP buffersize is now established when creating dispatch manager
  and is always set to 4096.

- Set up the default port range in dispatchmgr before setting the magic
  number.

- Magic is not set until dispatchmgr is fully created.

lib/dns/tests/resolver_test.c

@@ -58,7 +58,7 @@ _setup(void **state) {
 
 	isc_sockaddr_any(&local);
 	result = dns_dispatch_getudp(dispatchmgr, socketmgr, taskmgr, &local,
-				     4096, 100, 100, 100, 500, 0, &dispatch);
+				     100, 100, 100, 500, 0, &dispatch);
 	assert_int_equal(result, ISC_R_SUCCESS);
 
 	return (0);