Run startup_cmd in new session and kill the entire group

When a user's startup_cmd is a little more complex, e.g. a shell script,
and forks off several processes, then killing only the main child pid
might leave unwanted processes behind on exit. Not all children will
notice when their parent or the compositor has quit.

To fix this, put startup_cmd into its own session and process group, and
kill the entire group on exit.

dwl.c

@@ -674,7 +674,7 @@ cleanup(void)
 #endif
 	wl_display_destroy_clients(dpy);
 	if (child_pid > 0) {
-		kill(child_pid, SIGTERM);
+		kill(-child_pid, SIGTERM);
 		waitpid(child_pid, NULL, 0);
 	}
 	wlr_xcursor_manager_destroy(cursor_mgr);
@@ -2141,6 +2141,7 @@ run(char *startup_cmd)
 		if ((child_pid = fork()) < 0)
 			die("startup: fork:");
 		if (child_pid == 0) {
+			setsid();
 			dup2(piperw[0], STDIN_FILENO);
 			close(piperw[0]);
 			close(piperw[1]);