Open doors when right-clicking a door with a door.

And similarly, if we wield a door and right click any node
that has an on_rightclick() handler, call the handler
instead.

Just to be on the safe side, assure that none of this
code runs when right-clicking an entity or player, which
would likely crash the server.

Fold in PR #831 as well - prevent server crash on door
place on unknown blocks, by @tenplus1.

mods/doors/init.lua

@@ -185,17 +185,27 @@ function doors.register(name, def)
 		on_place = function(itemstack, placer, pointed_thing)
 			local pos = nil
 
+			if not pointed_thing.type == "node" then
+				return itemstack
+			end
+
 			local node = minetest.get_node(pointed_thing.under)
-			if minetest.registered_nodes[node.name].buildable_to then
+			local def = minetest.registered_nodes[node.name]
+			if def and def.on_rightclick then
+				return def.on_rightclick(pointed_thing.under,
+						node, placer, itemstack)
+			end
+
+			if def and def.buildable_to then
 				pos = pointed_thing.under
 			else
 				pos = pointed_thing.above
 				node = minetest.get_node(pos)
-			end
-
-			if not minetest.registered_nodes[node.name].buildable_to then
+				def = minetest.registered_nodes[node.name]
+				if not def or not def.buildable_to then
 					return itemstack
 				end
+			end
 
 			local above = { x = pos.x, y = pos.y + 1, z = pos.z }
 			if not minetest.registered_nodes[minetest.get_node(above).name].buildable_to then