Make base64 encoding/decoding part of rpmio public API

Make base64 encoding/decoding part of rpmio public API

- Base64 is present in headers and all, it's only reasonable that
  our API users have access to this functionality without having
  to link to other libraries. Even if we didn't want to carry the
  implementation forever in our codebase, we should provide a wrapping
  for this (much like the other crypto stuff) for the reason stated above.
- A bigger issue is that our dirty little (badly hidden) secret was using
  non-namespaced function names, clashing with at least beecrypt. And we
  couldn't have made these internal-only symbols even on platforms that
  support it, because they are used all over the place outside rpmio.
  So... rename the b64 functions to rpmLikeNamingStyle and make 'em public.
  No functional changes, just trivial renaming despite touching numerous
  places.