Remove hdb_entry_ex and revert to the original design of hdb_entry (except with
an additional context member in hdb_entry which is managed by the free_entry
method in HDB).
Add some notes about the KDC plugin API contract, and require plugins to
explicitly indicate which version of the API they support (remove the macro
alias for the current version).
Rename the "windc" plugin API to the more general "kdc" plugin API, for two
reasons: the Heimdal KDC uses the Windows PAC even when not emulating a domain
controller, and the plugin API has accreted methods that are not specific to
emulating a domain controller (such as referral_policy and finalize_reply).
