GPS and FCS
dr
Lots of changes to the GPS system - handling of GPS units with magnetic
heading reports, software calculation of heading for those without, storing
of the heading in the gpsxml file.  Fixing the compass should happen soon.

The FCS bytes now only get trimmed off card_prism2 and card_prism2_avs, this
should keep it from corrupting packets from orinoco (and other) which don't
put a FCS on the packet.