Reset dig exit code after a TCP connection is established

The "exitcode" variable is set to 9 if a TCP connection fails, but is
not reset to 0 if a subsequent TCP connection succeeds.  This causes dig
to return a non-zero exit code if it succeeds in getting a TCP response
after a retry.  Fix by resetting "exitcode" to 0 if connect_done()
receives an event with the "result" field set to ISC_R_SUCCESS.

(cherry picked from commit deb3b85cb2)

bin/dig/dighost.c

@@ -3526,6 +3526,7 @@ connect_done(isc_task_t *task, isc_event_t *event) {
 		UNLOCK_LOOKUP;
 		return;
 	}
+	exitcode = 0;
 	if (keep_open) {
 		if (keep != NULL)
 			isc_socket_detach(&keep);