Problem: zmq_bind IPv4 fallback still tries IPv6

Solution: if opening an IPv6 TCP socket fails because IPv6 is not
available, try to open an IPv4 socket instead when creating and
binding a TCP endpoint.

src/tcp_listener.cpp

@@ -184,13 +184,13 @@ int zmq::tcp_listener_t::set_address (const char *addr_)
 #endif
 
     //  IPv6 address family not supported, try automatic downgrade to IPv4.
-    if (address.family () == AF_INET6
+    if (s == -1 && address.family () == AF_INET6
     && errno == EAFNOSUPPORT
     && options.ipv6) {
-        rc = address.resolve (addr_, true, true);
+        rc = address.resolve (addr_, true, false);
         if (rc != 0)
             return rc;
-        s = ::socket (address.family (), SOCK_STREAM, IPPROTO_TCP);
+        s = open_socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
     }
 
 #ifdef ZMQ_HAVE_WINDOWS