Unbreak lua's base64 encode/decode extensions

Unbreak lua's base64 encode/decode extensions

- Commit 70f063cb773bedb7d336429d9bc8ed1d4e5d18f4 accidentally
  changed lua's base64 encode/decode interface too, ugh. Dangers of
  search-and-replace... Only the function name string exported to
  lua matters but renaming the internal functions back as well
  for naming consistency.