fabric: trivial param rename

Make it clear that this version parameter is the version of the FI API
itself (vs. the version of a given provider).
fi_register_provider: check provider version

If multiple providers register with the same name, keep only the one
with the highest version number.  Use the same format version number
as FI_VERSION.

Fixes #196.
Merge pull request #209 from jsquyres/pr/dl-versioning

Add provider versioning