Allow multi-hop SAM-2 exchanges

Prior to 1.11, it was possible to do SAM-2 preauth exchanges with
multiple hops by sending repeated preauth-required errors with
different challenges (which is not the way multi-hop exchanges are
described in RFC 6113, but it can still work).  This stopped working
when SAM-2 was converted to a built-in module.  Make it work again.

ticket: 7571 (new)
target_version: 1.11.1
tags: pullup