[review] Use glib functions for base64 decoding/encoding

This fixes several coverity warnings about 'tainted data index sink' and a fixme about thread safety in the old base64_decode implementation.

Had to adapt the code that used base64_encode_real:

 - oauth.c: different character set order, but it's for the nonce so it doesn't matter
 - libyahoo2.c: used as part of the auth, changes "+/=" into "._-". Fixed by encoding first the usual way through glib, then replacing.

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The only thing that needs review here, IMO, is the to_y64 function of libyahoo2.c. It works, just looking for opinions on the way I did that char replacement. I think this is cleaner than a switch(), but eh.