Fix getting screensize on OpenBSD

OpenBSD doesn't have 'stty -F'; an alternative is to use bash's native
'checkwinsize' shell option to populate $LINES and $COLUMNS.

Signed-off-by: Anirudh Oppiliappan <[e-mail]>

shlide

···
     # Get screen size.
     case "$OSTYPE" in
         "darwin"*) read -r LINES COLUMNS < <(stty -f /dev/tty size) ;;
+        "openbsd"*) shopt -s checkwinsize; (:;:) ;;
         *) read -r LINES COLUMNS < <(stty -F /dev/tty size) ;;
     esac