window: Fix the north/south cursor position when keyboard resizing

We got these backwards...
common: Make UNKNOWN a flag, not a combination of all directions

This was the original intention, but it was thought to be easier
to mark this as a combination of all directions. It turned out to
instead cause subtle bugs since code that blindly checked & DIR_WEST or
similar turned out to get it wrong when it was UNKNOWN, so just make
it an explicit flag.

This fixes the cursor appearing in the wrong place when starting a
keyboard resize.