scripts/kconfig/nconf: fix editing long strings

The original dialog_inputbox doesn't work with longer than prompt_width
strings, here fixed it in this way:
1) add variable cursor_form_win to record cursor of form_win,
   keep its value always between [0, prompt_width-1];
   reuse the original cursor_position as cursor of the string result,
   use (cursor_position-cursor_form_win) as begin offset to show part of
   the string in form_win;

Signed-off-by: Cheng Renquan <crquan@gmail.com>
Cc: Arnaud Lacombe <lacombar@gmail.com>
Cc: Nir Tzachar <nir.tzachar@gmail.com>