Make 'ghc --show-iface' give a reasonable error message on old
interface files again.  We previously disabled the version check for
--show-iface so that you could run --show-iface on a profiled
interface file, but this disabled too much error checking.  Really we
just want to disable the 'way' check, not the whole version check.

HEADS UP: interface format changed.  Recompile libraries.