Default almost all node meta fields to private.

Trying to isolate server performance issues, it was pointed out
that clients are spending a LOT of time receiving and rebuilding
mapblock meshes, and that updating node meta triggers this, even
when that meta is not relevant to the rendering of nodes.

Rubenwardy added a suggestion to change this in the engine:
	https://github.com/minetest/minetest/issues/10127
This patch implements this suggestion at the game level, since
in NodeCore's case we already were making the assumption that
nearly all metadata fields were private.

mods/nc_api/init.lua

@@ -81,6 +81,7 @@ end
 
 include("compat_vector")
 include("issue9043")
+include("nodemetahack")
 
 include("util_misc")
 include("util_hookmeta")

mods/nc_api/nodemetahack.lua

@@ -0,0 +1,35 @@
+-- LUALOCALS < ---------------------------------------------------------
+local getmetatable, minetest, pairs
+    = getmetatable, minetest, pairs
+-- LUALOCALS > ---------------------------------------------------------
+
+local publicfields = {
+	formspec = true,
+	infotext = true
+}
+
+local function hook(meta)
+	for k, v in pairs(meta) do
+		if k:sub(1, 4) == "set_" then
+			meta[k] = function(data, name, ...)
+				if not publicfields[name] then
+					data:mark_as_private(name)
+				end
+				return v(data, name, ...)
+			end
+		end
+	end
+end
+
+local rawmeta = minetest.get_meta
+function minetest.get_meta(...)
+	local raw = rawmeta(...)
+	if raw then
+		local meta = getmetatable(raw)
+		if meta then
+			hook(meta)
+			minetest.get_meta = rawmeta
+		end
+	end
+	return raw
+end