Add path to UNIXSocket created by UNIXServer

[straight-shoota]

A UNIXServer creates an instance of UNIXSocket for every incoming socket connection but unlike client sockets, the values for local_address and remote_address were missing. This PR adds them.

UNIXServer.open(path) do |server|
  server.local_address.path     # => path

  UNIXSocket.open(path) do |client|
    client.local_address.path   # => path
    client.remote_address.path  # => path

    server.accept do |sock|
      sock.local_address.path   # => path (previously "")
      sock.remote_address.path  # => path (previously "")
    end
  end
end

There were specs to explicitly check that local_address and remote_address on a server UNIXSocket would be "" but I don't think there is any reason for this and they should in fact return the path the server listens to.