polkit: rename bus_verify_polkit() to bus_test_polkit() and make it strictly non...

polkit: rename bus_verify_polkit() to bus_test_polkit() and make it strictly non-interactive

Interactive authorization should only happen asynchronously, hence
disallow it in synchronous bus_verify_polkit(), and rename it to
bus_test_polkit(). This way even if the bus message header asks for
interactive authorization, we'll ask for non-interactive authorization
which is actually the desired behaviour if CanSuspend, CanHibernate and
friends, which call this function.