Teach retry_listener about "auto" ports.

Otherwise, it will just immediately close any port declared with "auto"
on the grounds that it wasn't configured.  Now, it will allow "auto" to
match any port.

This means FWIW if you configure a socks port with SocksPort 9999
and then transition to SocksPort auto, the original socksport will
not get closed and reopened.  I'm considering this a feature.

src/or/connection.c

@@ -1772,10 +1772,23 @@ retry_listeners(int type, config_line_t *cfg,
             if (!parse_addr_port(LOG_WARN,
                                  wanted->value, &address, NULL, &port)) {
               int addr_matches = !strcasecmp(address, conn->address);
+              int port_matches;
               tor_free(address);
-              if (! port)
-                port = port_option;
-              if (port == conn->port && addr_matches) {
+              if (port) {
+                /* The Listener line has a port */
+                port_matches = (port == conn->port);
+              } else if (port_option == CFG_AUTO_PORT) {
+                /* The Listener line has no port, and the Port line is "auto".
+                 * "auto" matches anything; transitions from any port to
+                 * "auto" succeed. */
+                port_matches = 1;
+              } else {
+                /*  The Listener line has no port, and the Port line is "auto".
+                 * "auto" matches anything; transitions from any port to
+                 * "auto" succeed. */
+                port_matches = (port_option == conn->port);
+              }
+              if (port_matches  && addr_matches) {
                 line = wanted;
                 break;
               }