Problem: size of zmq_msg_t is not known to FFI wrappers

Solution: add a ZMQ_MSG_T_SIZE context read-only option so that
wrappers can call zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) to get the
size at runtime.

src/ctx.cpp

@@ -301,6 +301,9 @@ int zmq::ctx_t::get (int option_)
     else
     if (option_ == ZMQ_MAX_MSGSZ)
         rc = max_msgsz;
+    else
+    if (option_ == ZMQ_MSG_T_SIZE)
+        rc = sizeof (zmq_msg_t);
     else {
         errno = EINVAL;
         rc = -1;