Added support and autodetection of MCP4151 digital pot.
If MCP4151 is detected then its wiper resolution is halved
to match the MCP4131 wiper resolution.
MCP4151-103 is now a drop in replacement for MCP4131-103.
New features:
* unified socket for 20 pin and 24 devices (including GAL20V8)
* variable VPP via digi pot
* VPP measurement via A0 ADC (used for VPP calibration)