Add "Emulate3Buttons" support to the X68k Xorg based monolithic server.

Based on "Emulate3Buttons" functions in the xf86-input-mouse driver,
with various refactoring and cleanups:
- rewrite a magic three-dimensional array for a finate state machine
with proper named structures etc.
- separate 3 button emulation functions from other stuff

See my post on port-x68k@ (and tech-x11@) for more details:
https://mail-index.netbsd.org/port-x68k/2021/02/05/msg000074.html