When reading netlist or updating board from schematic, if the
changes leave a zone with a net with no pads then change the
zone's net to the new net of one of it's connections.
Also improves update-board-from-schematic's dry run reporting
to include zone nets and single-pad nets.
Fixes: lp:1609401
* https://bugs.launchpad.net/kicad/+bug/1609401