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.

@shefty and @pmmccorm likely care about this pull request.  Mentioning them just to ensure they get an email/notified about it.