(This is basically bin/named/server.c:base64_cstring_tobuffer(), which I
will remove in another revision that will not need to be pulled up.  I also
would like to change isc_base64_totext() and isc_base64_tobuffer() to be
isc_base64_encoderegion() and isc_base64_decodelexer(), unless there are
exceptions.  I find their existing names to be quite confusing with regard to
what translation function each performs.)