Several improvements to the Shield

changes:
1. removed the limitation to block while pointing a node wielding an item
2. check whether the player is pointing to a node defined with right-click Formspec (to avoid the formspec bug while blocking) and whether the player is wielding a node (to avoid continous node placing not functioning) before blocking
3. decreased the interval between right click and block

@@ -254,0 +256,4 @@
+end
+
+local function is_rmb_conflicting_node(nodename)
+	nodedef = minetest.registered_nodes[nodename]