Only let mesh->self be reachable when the mesh is started.

Only let mesh->self be reachable when the mesh is started.

This ensures meshlink_node_get_reachability(mesh->self) returns true only
if the mesh has been started. It also handles reachability of self in
graph.c just like any other node. This means there will now also be a
node status callback generated when the mesh is started and stopped.