Use KCM_OP_RETRIEVE in KCM client

Use KCM_OP_RETRIEVE in KCM client

In kcm_retrieve(), try KCM_OP_RETRIEVE.  Fall back to iteration if the
server doesn't implement it, or if we can an answer incompatible with
KRB5_TC_SUPPORTED_KTYPES.

In kcmserver.py, implement partial decoding for creds and cred tags so
that we can do a basic principal name match.

ticket: 8997 (new)