Debug accounting menu. Can now edit product account if already set.

htdocs/accountancy/admin/account.php

htdocs/accountancy/admin/defaultaccounts.php

htdocs/accountancy/admin/productaccount.php

htdocs/accountancy/class/html.formventilation.class.php

htdocs/core/lib/functions.lib.php

htdocs/langs/en_US/accountancy.lang

htdocs/langs/en_US/products.lang

htdocs/product/class/product.class.php