build: if linking with ncurses fails, try curses (for NetBSD)

build: if linking with ncurses fails, try curses  (for NetBSD)

NetBSD doesn't use ncurses, but instead has its own version of curses.
So, if we don't find ncurses during configure, try linking against
curses.

On NetBSD, we must include limits.h to find CHAR_MAX.

On OpenBSD, we must include sys/socket.h to get sockaddr_storage.