ui/ncurses: in lockdown ensure system reboot in ncurses menu exit

ui/ncurses: in lockdown ensure system reboot in ncurses menu exit

In a lockdown situation in the ncurses menu there is a switch to replace
the 'Exit to shell' option with 'Reboot', so the intent seems to be to
not allow the user the option to exit to shell in a lockdown situation.
However the associated foreced reboot logic is in the process atexit so
is only triggered when completely exiting the menu system.  The default
menu item logic to exit to shell is still in place though so the menu
exit never occurs and shell access is still available.
Add a switch to a different menu exit callback to force a menu abort
using the same mechanism as a signal in lockdown situations so the shell
can never be entered.  This also affects the 'x' or esc shortcut keys.

Signed-off-by: Brett Grandbois <brett.grandbois@opengear.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>