Cleanup: base64 coder de-duplication and upgrade

Cleanup: base64 coder de-duplication and upgrade

Markus Moeller has re-implemented several of the coder functions for use
by Kerberos helpers.

This patch seeks to de-duplicate them and combine the resulting code
back into the libmiscencoding.la "base64.h" implementation.

Changes include:

  * old function API renamed to old_*() and existing code update to use
the names. Some code has been updated to use the new API.

  * new estimator base64_encode_len()/base64_decode_len() functions added
to provide details of much much buffer space the output will require.

  * new API encoder and decoder functions added which accept caller
provided buffers and encode/decode an arbitrary string into them.

  * also fixes a bug where if the input text or output buffer was too
short the coder functions would crop a few bytes off the end of the result.
Noticable in Kerberos where token lengths are not fixed length.

Some optimizations have been added by myself over and above Markus changes:

  * optimized to short-circuit on several more variations of empty input
and nil result buffer.

  * sub-loop optimizations added to reduce the number of if() calls made
by the new code.

  * split encoder into terminated (C-str) and non-terminated variants.

Also documented both old and new APIs functions.