Move the pipe termination code to socket_base_t

So far, the pipe termination code was spread among socket type
classes, fair queuer, load balancer, etc. This patch moves
all the associated logic to a single place.

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

src/sub.hpp

@@ -37,7 +37,7 @@ namespace zmq
 
         int xsetsockopt (int option_, const void *optval_, size_t optvallen_);
         int xsend (class msg_t *msg_, int options_);
-	bool xhas_out ();
+        bool xhas_out ();
 
     private: