Replace ncurses by curses.

Replace ncurses by curses.

Ncurses is not available on all platforms, and para_gui should work
with any curses implementation.

So this patch replaces the --with-ncurses-headers and
--with-ncurses-libs configure options by --with-ncurses-headers and
--with-ncurses-libs, and makes configure check for libcurses instead
of libncurses. Moreover, the para_gui executable is now linked against
libcurses rather than libncurses.

On linux systems, this should be a no-op.