#4400, #5462: Move BASE64 helpers to crypto-utils

On a way to factoring out OpenSSL support to a standalone file to ease
addition of other crypto libraries support in the future, move helpers
providing BASE64 encoding and decoding to crypto-utils.{c,h}. OpenSSL-
related functionality is moved to crypto-utils-openssl.c.

Add new functions to be implemented by crypto backends:
* tr_base64_encode_impl - encode from binary to BASE64,
* tr_base64_decode_impl - decode from BASE64 to binary.

Change `tr_base64_encode` and `tr_base64_decode` functions to expect
non-negative input data length which is considered real and never adjusted.
To process null-terminated strings (which was achieved before by passing 0
or -1 as input data length), add new `tr_base64_encode_str` and
`tr_base64_decode_str` functions which do not accept input data length as
an argument but calculate it on their own.


git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14359 f4695dd4-2c0a-0410-b89c-da849a56a58e