ibverbs: rename provider-specific symbols to be "fi_ibv_*"

This clearly delineates between symbols here in this provider and
symbols that belong to libibverbs.

As a bonus side-effect, some provider symbols that began with "__"
(which are in the compiler's namespace) were eliminated.
Merge pull request #119 from jsquyres/topic/rename-ibv-to-fi_ibv

ibverbs: rename provider-specific symbols to be "fi_ibv_*"