nu-drop-overriding-map

Allows to remove a previously added map to
minor-mode-overriding-map-alist

This, in order not to have to mess up with nu-state, which
remains globally true.

This will make term feasible.
helm : advise to activate mode
Lay foundation to work on term

Switch, according to submode, which map to make overriding.
Now we "just" have to define these, which will be
the real interesting work...