Forward-compatibility

This enables a nicer warning on files advertising a newer opam-version than the exe,
enabling for those "permissive mode" that will parse what is compatible and silently
ignore what it doesn't understand (__the whole field__; granularity could be improved)

Still fails with newer MAJOR version number, or if $OPAMROOT/config advertises a newer
version

ref: #1662
Ensure repo redirections are'nt broken by opam-version checks
'make fast' fix
Merge pull request #1680 from OCamlPro/forward-compat

Attempt to improve forward-compat