Added test and doc section for ZMQ_SHARED message flag

doc/zmq_msg_get.txt

@@ -29,6 +29,10 @@ allows application to retrieve the remote endpoint via 'getpeername(2)'. Be
 aware that the respective socket might be closed already, reused even. 
 Currently only implemented for TCP sockets.
 
+*ZMQ_SHARED*::
+Indicates that a message has been copied and MAY share underlying storage
+with another copy of this message.
+
 RETURN VALUE
 ------------
 The _zmq_msg_get()_ function shall return the value for the property if

tests/test_msg_flags.cpp

@@ -65,6 +65,26 @@ int main (void)
     more = zmq_msg_more (&msg);
     assert (more == 0);
 
+    // Test ZMQ_SHARED property
+    zmq_msg_t msg_a;
+    rc = zmq_msg_init_size(&msg_a, 1024); // large enough to be a type_lmsg
+    assert (rc == 0);
+
+    // Message is not shared
+    rc = zmq_msg_get(&msg_a, ZMQ_SHARED);
+    assert (rc == 0);
+
+    zmq_msg_t msg_b;
+    rc = zmq_msg_init(&msg_b);
+    assert (rc == 0);
+
+    rc = zmq_msg_copy(&msg_b, &msg_a);
+    assert (rc == 0);
+
+    // Message is now shared
+    rc = zmq_msg_get(&msg_b, ZMQ_SHARED);
+    assert (rc == 1);
+
     //  Deallocate the infrastructure.
     rc = zmq_close (sc);
     assert (rc == 0);