Userland+Kernel: Set shutdown/reboot to only be run by the phys group

Author: https://github.com/thatlittlegit
Commit: https://github.com/SerenityOS/serenity/commit/ab9e5755bae
Pull-request: https://github.com/SerenityOS/serenity/pull/1281

Kernel/build-root-filesystem.sh

@@ -111,8 +111,12 @@ else
     find ../Userland/ -type f -executable -exec cp {} mnt/bin/ \;
 fi
 chown 0:$wheel_gid mnt/bin/su
+chown 0:$phys_gid mnt/bin/shutdown
+chown 0:$phys_gid mnt/bin/reboot
 chmod 4750 mnt/bin/su
 chmod 4755 mnt/bin/ping
+chmod 4750 mnt/bin/reboot
+chmod 4750 mnt/bin/shutdown
 echo "done"
 
 printf "installing applications... "