nconf: use function calls instead of ncurses' variables LINES and COLS

According to the documentation [1], LINES and COLS are initialized by
initscr(); it does not say anything about the behavior when windows are
resized.

Do not rely on the current implementation of ncurses that updates
these variables on resize, but use the propper function calls or macros
to get window dimensions.

The use of the variables in main() was OK, but for the sake of
consistency it was modified to use the macro getmaxyx().

[1] ncurses(3X)

Signed-off-by: Dirk Gouders <dirk@gouders.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: declare 'lines' and 'columns' on a single line]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>