Pcbnew: fix issues and potential issues when reading board files and mainly old board files, and enhance the DRC tests and Delete Single Pad Net option when reading a netlist:
* Delete Single Pad Net option does not delete the net if a zone use this net (i.e. is attached to this pad).
* pcb_parser accept now negative netcodes in zones (can happen with old files, which previously could crash Pcbnew)
* pcb_parser accept now files with incorrect or missing net count  (can happen with old files, which previously could crash Pcbnew)
* if a zone has a non-existent net name it now keep this net name, and DRC detect it (previously, the net name was lost, and the DRC did not tected this issue).
* Drc test: now detect a "dead" net, i.e. a net with 0 pads, but still used by a zone. It happens easily after a schematic modification, when a net disappears or is renamed.