Allow spawned processes to watch for the parent's death

[kolayne]

Use case

On Linux, programs may request to be notified of their parent's death with prctl (PR_SET_PDEATHSIG). For example, on my previous WM I used to auto-start some tools that work in the background without displaying anything and terminate automatically on the window manager exit.

However, it only makes sense if the spawned processes remain children of the process who is logically their parent.

Current behavior

Hyprland spawns new processes as grand-children, e.g.:

Hyprland/src/managers/KeybindManager.cpp

Lines 959 to 995 in 5e96fac

	pid_t child, grandchild;
	child = fork();
	if (child < 0) {
	Debug::log(LOG, "Fail to create the first fork");
	return 0;
	}
	if (child == 0) {
	// run in child
	g_pCompositor->restoreNofile();
	sigset_t set;
	sigemptyset(&set);
	sigprocmask(SIG_SETMASK, &set, nullptr);
	grandchild = fork();
	if (grandchild == 0) {
	// run in grandchild
	for (auto const& e : HLENV) {
	setenv(e.first.c_str(), e.second.c_str(), 1);
	}
	setenv("WAYLAND_DISPLAY", g_pCompositor->m_wlDisplaySocket.c_str(), 1);
	int devnull = open("/dev/null", O_WRONLY | O_CLOEXEC);
	if (devnull != -1) {
	dup2(devnull, STDOUT_FILENO);
	dup2(devnull, STDERR_FILENO);
	close(devnull);
	}
	execl("/bin/sh", "/bin/sh", "-c", args.c_str(), nullptr);
	// exit grandchild
	_exit(0);
	}
	write(pipeSock[1].get(), &grandchild, sizeof(grandchild));
	// exit child
	_exit(0);
	}

This way, spawned processes are immediately orphaned and reparented.

Desired behavior

A more logical/natural behavior would be to spawn processes as direct children of the WM.

Implementation

Presumably, processes are spawned as grandchildren to lift the burden of dealing with zombies. However, Hyprland doesn't seem to ever wait on its children to terminate (except for this grandchildren case). Thus, we could just set SA_NOCLDWAIT at launch of Hyprland and then have processes spawned as direct children without any more code to handle their termination.

[kolayne]

Done: #11735