Restore LuaATC autocouple interface accidentally lost in 74a2199

During rebase, this part of the autocouple feature must have been accidentally lost

advtrains_luaautomation/atc_rail.lua

@@ -96,6 +96,18 @@ function r.fire_event(pos, evtdata, appr_internal)
 			if not train_id then return false end
 			train.is_shunt = true
 		end,
+		unset_shunt = function()
+			if not train_id then return false end
+			train.is_shunt = nil
+		end,
+		set_autocouple = function ()
+			if not train_id then return false end
+			train.autocouple = true			
+		end,
+		unset_autocouple = function ()
+			if not train_id then return false end
+			train.autocouple = nil
+ 		end,
 		set_line = function(line)
 			if type(line)~="string" and type(line)~="number" then
 				return false