前台增加會員時判斷帳號是否已存在，不然退回上一頁

Conflicts:
	cart.php
	cart/index.php
	lang/chs-utf8.php
	lang/cht-utf8.php
	lang/eng-utf8.php
	lang/fre-utf8.php
	lang/ger-utf8.php
	lang/ind-utf8.php
	lang/jap-utf8.php
	lang/kor-utf8.php
	lang/por-utf8.php
	lang/rus-utf8.php
	lang/spa-utf8.php
	lang/tur-utf8.php
	libs/libs-main.php
	member.php