fix mistakes in exchange.lua and fix style in exchange_machine.lua

exchange.lua

@@ -819,7 +819,7 @@ function ex_methods.buy(self, p_name, ex_name, item_name, amount, rate)
 				return false, log2_err
 			end
 
-			break
+			remaining = 0
 		end
 
 		if remaining == 0 then break end
@@ -983,8 +983,7 @@ function ex_methods.sell(self, p_name, ex_name, item_name, amount, rate)
 				return false, log2_err
 			end
 
-			break
-			--revenue = revenue + remaining * row.Rate
+			remaining = 0
 		end
 
 		if remaining == 0 then break end

exchange_machine.lua

@@ -161,8 +161,8 @@ local function mk_main_fs(p_name, new_item, err_str, success)
 end
 
 
-local function show_main(p_name, ...)
-	minetest.show_formspec(p_name, main_form, mk_main_fs(p_name, ...))
+local function show_main(p_name, new_item, err_str, success)
+	minetest.show_formspec(p_name, main_form, mk_main_fs(p_name, new_item, err_str, success))
 end
 
 
@@ -220,6 +220,7 @@ end)
 
 
 minetest.register_on_joinplayer(function(player)
+	-- the inventory list name is "p_"..player_name
 	minetest.get_inventory({
 		type="detached",
 		name="global_exchange"