Serial mouse drivers

This adds support for serial mice (Microsoft and Mousesystems), so I could build a nice keyboard+mouse converter for my [Marquardt Military keyboard](http://deskthority.net/wiki/Marquardt_Military_Keyboard_7069.3111), which has an integrated trackball that uses the Mousesystems serial protocol.

The Microsoft mouse driver is **untested** -- I merely implemented it by accident as I did not know which protocol the trackball uses.