Problem: peer can close connection before SO_NOSIGPIPE is set

Solution: setsockopt returns EINVAL if the connection was closed by
the peer after the accept returned a valid socket. This is a valid
network error and should not cause an assert.
To handle this we have to extract the setsockopt from the stream
engine, as there's no clean way to return an error from the
constructor. Instead, try to set this option before creating the
engine in the callers, and return immediately as if the accept
had failed to avoid churn. Do the same for the connect calls by
setting the option in open_socket, so that the option for that
case is set even before connecting, so there's no possible race
condition.
Since this has to be done in 4 places (tcp/ipc listener, socks
connecter and open_socket) add an utility function in ip.cpp.
Fixes #1442

src/stream_engine.cpp

@@ -132,13 +132,6 @@ zmq::stream_engine_t::stream_engine_t (fd_t fd_, const options_t &options_,
     }
 #endif
 
-#ifdef SO_NOSIGPIPE
-    //  Make sure that SIGPIPE signal is not generated when writing to a
-    //  connection that was already closed by the peer.
-    int set = 1;
-    rc = setsockopt (s, SOL_SOCKET, SO_NOSIGPIPE, &set, sizeof (int));
-    errno_assert (rc == 0);
-#endif
     if(options.heartbeat_interval > 0) {
         heartbeat_timeout = options.heartbeat_timeout;
         if(heartbeat_timeout == -1)