mxgrub: Restrict access to CLI and entry editing in submenu

Currently, it’s possible, after entering the submenu, to enter the CLI
by pressing the key *c*, or to edit entries by pressing *e*.

David found out, that adding the superuser and password line in the
submenu section allows to restrict the access in the submenu too.

mxgrub/mxgrub

@@ -274,6 +274,7 @@ else
 menuentry "$MARIUX_DEFAULT" --unrestricted { set chosen="$submenu>$MARIUX_DEFAULT" ; save_env chosen ; linux /boot/bzImage.x86_64 root=LABEL=root $KERNEL_PARAMETER ; initrd /boot/grub/initramfs.igz }
 
 submenu "$submenu" --unrestricted {
+$SUPERUSERS_LINE
 $kernellist
 }
        if [ \$chosen ]; then