nconfig: add search support

Remove the old hotkeys feature, and replace it by an interactive string
search.
From nconfig help:

Searching: pressing '/' triggers interactive search mode.
           nconfig performs a case insensitive search for the string
           in the menu prompts (no regex support).
           Pressing the up/down keys highlights the previous/next
           matching item. Backspace removes one character from the
           match string. Pressing either '/' again or ESC exits
           search mode. All other keys behave normally.

Miscellaneous other changes (including Rundy's and Justin's input).

Signed-off-by: Nir Tzachar <nir.tzachar@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>