bus-polkit: shortcut auth. after first denial

bus-polkit: shortcut auth. after first denial

A D-Bus/Varlink method can issue PolicyKit auth. requests for multiple
actions; in this case the method is expected to fail on the first one
that is not allowed. This is enforced by asserts in
async_polkit_read_reply(), but that's a wrong place for the check for
two reasons:

1. it doesn't allow to get a meaningful stack trace;
2. sending the query to polkit is already a pointless exercise.

Let's do the check in *_verify_polkit_async_full() and don't send
anything to PolicyKit in that case.

Inspired by https://bugzilla.redhat.com/show_bug.cgi?id=2349594 .