Use single port for creating signalers on Windows

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/config.hpp

@@ -79,7 +79,11 @@ namespace zmq
         clock_precision = 1000000,
 
         //  Maximum transport data unit size for PGM (TPDU).
-        pgm_max_tpdu = 1500
+        pgm_max_tpdu = 1500,
+
+        //  On some OSes the signaler has to be emulated using a TCP
+        //  connection. In such cases following port is used.
+        signaler_port = 5905
     };
 
 }