audit: convert status version to a feature bitmap

The version field defined in the audit status structure was found to have
limitations in terms of its expressibility of features supported.  This is
distict from the get/set features call to be able to command those features
that are present.

Converting this field from a version number to a feature bitmap will allow
distributions to selectively backport and support certain features and will
allow upstream to be able to deprecate features in the future.  It will allow
userspace clients to first query the kernel for which features are actually
present and supported.  Currently, EINVAL is returned rather than EOPNOTSUP,
which isn't helpful in determining if there was an error in the command, or if
it simply isn't supported yet.  Past features are not represented by this
bitmap, but their use may be converted to EOPNOTSUP if needed in the future.

Since "version" is too generic to convert with a #define, use a union in the
struct status, introducing the member "feature_bitmap" unionized with
"version".

Convert existing AUDIT_VERSION_* macros over to AUDIT_FEATURE_BITMAP*
counterparts, leaving the former for backwards compatibility.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[PM: minor whitespace tweaks]
Signed-off-by: Paul Moore <pmoore@redhat.com>