instead of active_connection->invisible.
(login): Likewise.
(logout): Likewise.
(pepsi): Likewise.
(change_what_i_am_doing): Likewise.
(who_is_on): Likewise.
(who_is_on_ident): Likewise.
(who_is_on_old): Likewise.
(who_is_on_dynamic): Likewise.  Sessions with no user are always invisible.
(user_active): Set active_connection->flags.user_active_used.