Add etype-info2 to MORE_PREAUTH_DATA_REQUIRED

Add etype-info2 to MORE_PREAUTH_DATA_REQUIRED

A multi-round-trip preauth mechanism may require key information, but
not for the initial message from the client.  To support optimistic
preauth for such mechanisms, make the KDC include etype-info2
information in a MORE_PREAUTH_DATA_REQUIRED error if the client didn't
include a PA-FX-COOKIE in its request.

Add optimistic preauth support to the test preauth module and to
etinfo.c, and add a test case to t_etype_info.py to verify that
etype-info2 is included in the optimistic multi-hop scenario.

ticket: 8234 (new)