Close file descriptors on exec (issue 218)

When exec is executed to start a different process image old
0MQ file descriptors could stay open, thus blocking TCP ports
and alike. This patch should solve the problem.

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

src/tcp_listener.cpp

@@ -126,7 +126,7 @@ int zmq::tcp_listener_t::set_address (const char *addr_)
         return -1;
 
     //  Create a listening socket.
-    s = ::socket (address.family (), SOCK_STREAM, IPPROTO_TCP);
+    s = open_socket (address.family (), SOCK_STREAM, IPPROTO_TCP);
 #ifdef ZMQ_HAVE_WINDOWS
     if (s == INVALID_SOCKET)
         wsa_error_to_errno ();