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.