parse the ignore flag before all others
    
    The legacy implementation doesn't support "%+_", "%-_" etc. which were
    accepted by Benoît's patches. This creates a discrepancy (though not
    a regression) that is not necessarily desirable (and in any case makes
    fuzz-testing for difference return non-bugs). Long-term, I think that
    we're good with the previous, simpler structure where "_" must always
    be the first flag.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14827 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02