hdb: consolidate preauth audit event types

hdb: consolidate preauth audit event types

Instead of having distinct preauth success/failure events for different
mechanisms, have a single event; the mechanism can be disambiguated by querying
the HDB_REQUEST_KV_PA_NAME key.

Note: there is still an explicit event for long-term key-based success/failure
in order to help the backend implement lockout.

Audit failure (HDB_AUTH_EVENT_PREAUTH_FAILED) in the main preauth loop, rather
than in each mechanism. Success is still audited in the mechanism to allow
client pre-authentication success to be noted even if something subsequent
(e.g. encoding a reply, memory allocation) fails. The generic catch-all for
success remains.