[route parsing] make surf_network_resource_by_name public under name sg_routing_edge_...

[route parsing] make surf_network_resource_by_name public under name sg_routing_edge_by_name_or_null

* This function existed already, but were almost never used, its
  content being inlined in ways too much location.
* Use that function were relevant. That's much cleaner this way.
* Making it public is one step further to the C interface for routing
  creation interface (drop in to XML)