audit: clean up AUDIT_GET/SET local variables and future-proof API

Re-named confusing local variable names (status_set and status_get didn't agree
with their command type name) and reduced their scope.

Future-proof API changes by not depending on the exact size of the audit_status
struct and by adding an API version field.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>