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*.

According to Vladimir, [variables related to security have to be
exported][1].

[1]: https://lists.gnu.org/archive/html/grub-devel/2019-11/msg00128.html

mxgrub/mxgrub

@@ -257,6 +257,7 @@ sub update_grub_cfg {
 set timeout=5
 
 set superusers="root"
+export superusers
 password_pbkdf2 root grub.pbkdf2.sha512.10000.A1168F03CC3CD47F79848E949584EA624FF531B53611F61218DC5BAD760E767063A96E6A5CE6B3506BAC28C5A2F9A7D491BDA770CC233BC9F464F33F8524FC79.EE4EF2EB7B2B8C2DD313300396179F5484CAA011D7CBF2A3FF886A372A6FF2DBC3B1AE59E05687B294D4B8F4674DA55CCE18EDBEB2635E4B6F2BE7C4EBC8916A
 
 set default="$MARIUX_DEFAULT"