Base64 code cleanups.

The base64 encoder/decoder can use some love of the following sort:

• The code gated by USE_OPENSSL_BASE64 should probably be removed, since we haven't used OpenSSL's base64 decoder since 2007, and enabling it will do weird and broken things (Eg: Inputs that aren't broken up into into lines that are < 80 characters will fail to decode correctly).
• It would be really nice to have a Base64 encode that doesn't automatically add \n characters.
• Having a routine to get the encoded output size makes for cleaner code.