#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.

libtransmission/utils.h

@@ -287,22 +287,6 @@ char* tr_strdup_printf (const char * fmt, ...) TR_GNUC_PRINTF (1, 2)
 char * tr_strdup_vprintf (const char * fmt,
                           va_list      args) TR_GNUC_MALLOC;
 
-/**
- * @brief Translate a block of bytes into base64
- * @return a newly-allocated string that can be freed with tr_free ()
- */
-char* tr_base64_encode (const void * input,
-                        int          inlen,
-                        int        * outlen) TR_GNUC_MALLOC;
-
-/**
- * @brief Translate a block of bytes from base64 into raw form
- * @return a newly-allocated string that can be freed with tr_free ()
- */
-char* tr_base64_decode (const void * input,
-                        int          inlen,
-                        int        * outlen) TR_GNUC_MALLOC;
-
 /** @brief Portability wrapper for strlcpy () that uses the system implementation if available */
 size_t tr_strlcpy (char * dst, const void * src, size_t siz);