Don't log audit messages with UNKNOWN hostname

Don't log audit messages with UNKNOWN hostname

The `host` parameter to audit_log_acct_message() is documented as
follows:

      host - The hostname if known. If not available pass a NULL.

but we pass the string "UNKNOWN" in case we don't know the hostname.
Make sure we pass NULL instead.

This avoids having the audit system attempt to perform a DNS lookup on
the hostname "UNKNOWN", which tends to result in long delays when
attempting to login.