Improve base64 BIO correctness and error reporting

Also improve related documentation.

- The BIO_FLAGS_BASE64_NO_NL flag did not behave as advertised, only
leading and trailin

Improve base64 BIO correctness and error reporting

Also improve related documentation.

- The BIO_FLAGS_BASE64_NO_NL flag did not behave as advertised, only
leading and trailing, but not internal, whitespace was supported:

$ echo 'AA AA' | openssl base64 -A -d | wc -c
0

- Switching from ignored leading input to valid base64 input misbehaved
when the length of the skipped input was one more than the length of
the second and subsequent valid base64 lines in the internal 1k
buffer:

$ printf '#foo\n#bar\nA\nAAA\nAAAA\n' | openssl base64 -d | wc -c
0

- When the underlying BIO is retriable, and a read returns less than
1k of data, some of the already buffered input lines that could have
been decoded and returned were retained internally for a retry by the
caller. This is somewhat surprising, and the new code decodes as many
of the buffered lines as possible. Issue reported by Michał Trojnara.

- After all valid data has been read, the next BIO_read(3) should
return 0 when the input was all valid or -1 if an error was detected.
This now occurs in more consistently, but further tests and code
refactoring may be needed to ensure this always happens.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25253)

show more ...