Problem: test_large_msg kills my system temporarily

And I'm on a reasonably sized laptop. I think allocating INT_MAX
memory is dangerous in a test case.

Solution: expose this as a context option. I've used ZMQ_MAX_MSGSZ
and documented it and implemented the API. However I don't know how
to get the parent context for a socket, so the code in zmq.cpp is
still unfinished.

src/socket_base.hpp

@@ -245,7 +245,7 @@ namespace zmq
         void update_pipe_options(int option_);
 
         //  Socket's mailbox object.
-        i_mailbox* mailbox;
+        i_mailbox *mailbox;
 
         //  List of attached pipes.
         typedef array_t <pipe_t, 3> pipes_t;