We initialize a reverse map for base64 decoding using mongoc_b64_initialize_rmap. The function is not thread-safe, so if we plan to use it in SCRAM-SHA-1 authentication then we call the function in mongoc_init. We determine whether SCRAM-SHA-1 authentication is possible by checking if SSL is compiled in.

This is brittle; it's easy to miss the dependency between SSL and a base 64 decoder map, and accidentally use the map unsafely. Let's just always initialize it.