Let's add a proper validation function, since validation isn't entirely
trivial. Make use of it where applicable. Also make use of
AUDIT_SESSION_INVALID where we need a marker for an invalid audit
session.
The files are named too generically, so that they might conflict with
the upstream project headers. Hence, let's add a "-util" suffix, to
clarify that this are just our utility headers and not any official
upstream headers.