NETINFO_ITEM::m_Net and NETINFO_ITEM::m_Netname are const.

Changes to be verified:
- pcbnew/minimun_spanning_tree.cpp: It segfaults is m_Size == 0
- pcbnew/exporters/export_gencad.cpp: I removed the SetNetname() call, as it changes only the unconnected net and in the next line it returns if the net is unconnected. Still, I wonder if name for the unconnected net matters. What about tests that check if a net name is empty to decide if it is unconnected net or not.