formats: enable the legacy mode by default
    
    Given that there still remains a small incompatibility (typing of
    %(..%)), I decided to keep the legacy mode enabled for now. This means
    that any failure related to format can be traced to this
    incompatiblity (or unknown regressions), which will simplify the
    monitoring and handling of changes considerably. As soon as the %(..%)
    typing is generalized, we can turn the legacy mode off (or maybe
    simply add warnings for ignored formats).
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14841 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02