SEGZONE types were confusingly named PCB_ZONE_T. Zones in pcbnew are
now _only_ PCB_ZONE_AREA_T, so we name segzone types PCB_SEGZONE_T to be
clear.
This also removes processing of the SEGZONEs from connectivity
calculations.
SELECTION now holds EDA_ITEMs not BOARD_ITEMs so various places had to
change to casting the selected items to BOARD_ITEMs.
Fixed compilation warnings on clang (Tom)