Add chatcommand unregister and override API (#5076)

Introduces two functions to unregister and override chatcommands.
minetest.unregister_chatcommand("<name>") and
minetest.override_chatcommand("<name>", {<redifinition>})

builtin/game/chatcommands.lua

@@ -15,6 +15,24 @@ function core.register_chatcommand(cmd, def)
 	core.registered_chatcommands[cmd] = def
 end
 
+function core.unregister_chatcommand(name)
+	if core.registered_chatcommands[name] then
+		core.registered_chatcommands[name] = nil
+	else
+		core.log("warning", "Not unregistering chatcommand " ..name..
+			" because it doesn't exist.")
+	end
+end
+
+function core.override_chatcommand(name, redefinition)
+	local chatcommand = core.registered_chatcommands[name]
+	assert(chatcommand, "Attempt to override non-existent chatcommand "..name)
+	for k, v in pairs(redefinition) do
+		rawset(chatcommand, k, v)
+	end
+	core.registered_chatcommands[name] = chatcommand
+end
+
 core.register_on_chat_message(function(name, message)
 	local cmd, param = string.match(message, "^/([^ ]+) *(.*)")
 	if not param then

doc/lua_api.txt

@@ -2077,6 +2077,10 @@ Call these functions only at load time!
 ### Other registration functions
 * `minetest.register_chatcommand(cmd, chatcommand definition)`
     * Adds definition to minetest.registered_chatcommands
+* `minetest.override_chatcommand(name, redefinition)`
+    * Overrides fields of a chatcommand registered with register_chatcommand.
+* `minetest.unregister_chatcommand(name)`
+    * Unregisters a chatcommands registered with register_chatcommand.
 * `minetest.register_privilege(name, definition)`
     * `definition`: `"description text"`
     * `definition`: `{ description = "description text", give_to_singleplayer = boolean}`