This has most of the features needed to act as a kinit that uses GSS
APIs, specifically gss_acquire_cred_from() and gss_store_cred_into2().
It's missing some functionality, such as being able to drive prompts
from AS responses (if we add minor status codes for representing KDC
pre-auth proposals, then we do drive prompts, but we would have to
encode a lot of mechanism-specific knowledge into gsstool).
The point of this commit is to explore:
- GSS functionality for kinit-like actions
- credential store key/value pairs supported by the mechanisms
- document the credential store key/value pairs (in gsstool.1)
that might lead to further enhancements. But gsstool acquire-cred
is quite functional at this point!
