Pushing cffi-features symbols to *features*

- Renamed :cffi/no-foreign-funcall and :cffi/no-long-long to
  cffi-features:foreign-funcall and cffi-features:long-long respectively
  (with the opposite meanings of course).
- Also new features (in the cffi-features package): darwin, unix, windows,
  ppc32 and x86. More could be added. The cffi-sys backends are
  responsible for pushing these features.