internal: prevent early exit processes from being zombies (#11995)

Prevent `exec`/`exec-once` processes which terminate very early
(before Hyprland declares that it does not want to reap zombies)
from getting stuck as zombie processes.

hyprtester/test.conf

@@ -46,6 +46,7 @@ $menu = wofi --show drun
 # Autostart necessary processes (like notifications daemons, status bars, etc.)
 # Or execute your favorite apps at launch like this:
 
+exec-once = sleep 0  # Terminates very quickly
 # exec-once = $terminal
 # exec-once = nm-applet &
 # exec-once = waybar & hyprpaper & firefox