powermenu: Adapt for both archlinux and voidlinux

Keeps using the known systemctl commands for voidlinux environments (or
other environments that have access to the `systemctl` command).

For voidlinux it instead uses the commands `poweroff`, `reboot`, `zzz`
and `ZZZ` (which are the same script). Works fairly naively for now and
does no checking if the commands exist or work. Runs required commands
as sudo so ideally the user or group has access to a passwordless
implementation of the commands.

scripts/.local/bin/powermenu

@@ -58,12 +58,20 @@ result=$(printf "%s" "$menu" | $selector_program $selector_opts --prompt "$promp
 case "$result" in
 "Shutdown" | "$power_off_btn")
     if [ -n "$POWERMENU_SHUTDOWN_CMD" ]; then eval "$POWERMENU_SHUTDOWN_CMD"; else
-        systemctl poweroff
+        if [ -x /usr/bin/systemctl ]; then
+            systemctl poweroff
+        else
+            sudo poweroff
+        fi
     fi
     ;;
 "Reboot" | "$reboot_btn")
     if [ -n "$POWERMENU_REBOOT_CMD" ]; then eval "$POWERMENU_REBOOT_CMD"; else
-        systemctl reboot
+        if [ -x /usr/bin/systemctl ]; then
+            systemctl reboot
+        else
+            sudo reboot
+        fi
     fi
     ;;
 "Lockscreen" | "$lock_btn")
@@ -81,12 +89,20 @@ case "$result" in
     ;;
 "Suspend" | "$suspend_btn")
     if [ -n "$POWERMENU_SUSPEND_CMD" ]; then eval "$POWERMENU_SUSPEND_CMD"; else
-        systemctl suspend-then-hibernate
+        if [ -x /usr/bin/systemctl ]; then
+            systemctl suspend-then-hibernate
+        else
+            sudo zzz -H
+        fi
     fi
     ;;
 "Hibernate" | "$hibernate_btn")
     if [ -n "$POWERMENU_HIBERNATE_CMD" ]; then eval "$POWERMENU_HIBERNATE_CMD"; else
-        systemctl hibernate
+        if [ -x /usr/bin/systemctl ]; then
+            systemctl hibernate
+        else
+            sudo ZZZ
+        fi
     fi
     ;;
 esac