ui/ncurses: Add nc-auth and authenticate when required.

When the user tries to perform actions that require authentication a new
subscreen 'nc-auth' is launched which accepts a password and will send
an authentication request before performing the action.
This also adds a button in nc-config which launches an nc-auth screen
allowing the user to set or change the system password.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>