Add /killme chat command

Added as a new mod as it doesn't fit anywhere else.

This is a game play change, so it better suited to
Minetest Game rather than builtin

Fixes #857

mods/killme/init.lua

+minetest.register_chatcommand("killme", {
+	description = "Kill yourself to respawn",
+	func = function(name)
+		local player = minetest.get_player_by_name(name)
+		if player then
+			if minetest.setting_getbool("enable_damage") then
+				player:set_hp(0)
+				return true
+			else
+				for _, callback in pairs(core.registered_on_respawnplayers) do
+					if callback(player) then
+						return true
+					end
+				end
+
+				-- There doesn't seem to be a way to get a default spawn pos from the lua API
+				return false, "No static_spawnpoint defined"
+			end
+		else
+			-- Show error message if used when not logged in, eg: from IRC mod
+			return false, "You need to be online to be killed!"
+		end
+	end
+})