Add CHECK_FOR_INTERRUPTS() to the wait_pid() loop.

Though the one contemporary caller uses it in a limited way, this
function could loop indefinitely if pointed to an arbitrary PID.

src/test/regress/regress.c

@@ -870,7 +870,10 @@ wait_pid(PG_FUNCTION_ARGS)
 		elog(ERROR, "must be superuser to check PID liveness");
 
 	while (kill(pid, 0) == 0)
+	{
+		CHECK_FOR_INTERRUPTS();
 		pg_usleep(50000);
+	}
 
 	if (errno != ESRCH)
 		elog(ERROR, "could not check PID %d liveness: %m", pid);