* gds.el (gds-run-debug-server): Use variable

* gds.el (gds-run-debug-server): Use variable
gds-server-port-or-path instead of hardcoded 8333.
(gds-server-port-or-path): New.

* gds-server.el (gds-start-server): Change port arg to
port-or-path, to support Unix domain sockets.

* gds-client.scm (connect-to-gds): Try to connect by Unix domain
socket if TCP connection fails.

* gds-server.scm (run-server): Update to support listening on a
Unix domain socket.