LJ::make_authas_select — Given a u object and some options, determines which users the given user can switch to. If the list exists, returns a select list and a submit button with labels. Otherwise returns a hidden element.
u. A hashref of a row from the user table, sometimes called a "user object". Although only initially a row from the user table, gets a lot of tacked onto it from various other LJ API functions.
opts. Optional. Valid keys are: 'authas' - current user, gets selected in drop-down; 'label' - label to go before form elements; 'button' - button label for submit button; others - arguments to pass to LJ::get_authas_list.