Set and arrange propagation of thread safe sockets flag.

We use a distinct context initialisation function to specify
all sockets derived therefrom will be thread safe.

However the inheritance is done exclusively in the C interface.
This is not really correct, but it is chosen to minimise
interference with the existing C++ code, including any
construct or other calls within the C++ code base.
Semantically the C++ code should be unchanged,
physically some data structures and extra methods are
provided by they're only used from the C binding.

src/socket_base.cpp

@@ -874,6 +874,11 @@ void zmq::socket_base_t::extract_flags (msg_t *msg_)
     rcvmore = msg_->flags () & msg_t::more ? true : false;
 }
 
+void zmq::socket_base_t::set_thread_safe()
+{
+   thread_safe_flag = true;
+}
+
 void zmq::socket_base_t::lock() 
 {
    sync.lock();