mconf: 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 to get
window dimensions.

init_dialog() could make use of the variables, but for the sake of
consistency we do not change it's current use of the macro getmaxyx().

[1] ncurses(3X)

Signed-off-by: Dirk Gouders <dirk@gouders.net>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>