Sync with most changes in AS/TGS client from Apple's Heimdal-597.121.1
(opensource.apple.com).
Changes include:
- FAST support in TGS client
- Refactored pre-auth client to be more easily extensible
- Pin KDC host and AD site name in API calls
Note the completely refactored TGS client loop is not imported as that was
considered too intrusive.