Make fingerprint struct opaque outside fprint.c

Make fingerprint struct opaque outside fprint.c

- rpmfi cannot know anything about the storage, so rpmfiFpxIndex()
  cannot be... change it to rpmfiFps() which only returns the pointer
  we got from fpLookupList()
- Change fpCacheGetByFp() to assume it gets passed an array of fps,
  and take an additional index argument. Return the fingerprint
  pointer on success, NULL on not found to allow further operations
  on the fp without knowing its internals.