Add "fingersMap" option to Synaptics

[abbradar]

This allows one to remap taps -- useful for clickpads.

[puffnfresh]

I previously set tapButtons to false in my configuration:

services.xserver.synaptics.tapButtons = false;

Upgrading has broken that, probably because 118096f adds back in the TapButtons mapping.

[puffnfresh]

Worked around with:

services.xserver.synaptics.fingersMap = [ 0 0 0 ];