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/ctx.hpp

@@ -99,6 +99,10 @@ namespace zmq
             reaper_tid = 1
         };
 
+        // create thread safe sockets
+        void set_thread_safe();
+        bool get_thread_safe() const;
+
         ~ctx_t ();
     private:
 
@@ -151,6 +155,8 @@ namespace zmq
         zmq::socket_base_t *log_socket;
         mutex_t log_sync;
 
+        bool thread_safe_flag;
+
         ctx_t (const ctx_t&);
         const ctx_t &operator = (const ctx_t&);
     };