Audit the file-descriptor number argument for openat(2). Remove a comment
about the desirability of auditing the number, as it was in fact in the
wrong place (in the common path for open(2) and openat(2), and only the
latter accepts a file-descriptor argument). Where other ABIs support
openat(2), it may be necessary to do additional argument auditing as it is
not performed in kern_openat(9).

MFC after: 3 days
Sponsored by: DARPA, AFRL