fix startup_cmd SIGCHLD handler

Ignored handlers are not reset by exec() calls

dwl.c

@@ -1963,6 +1963,8 @@ run(char *startup_cmd)
 		if ((child_pid = fork()) < 0)
 			die("startup: fork:");
 		if (child_pid == 0) {
+			sa.sa_handler = SIG_DFL;
+			sigaction(SIGCHLD, &sa, NULL);
 			dup2(piperw[0], STDIN_FILENO);
 			close(piperw[0]);
 			close(piperw[1]);