basenc: auto pad base32 and base64 inputs when decoding

basenc: auto pad base32 and base64 inputs when decoding

Padding of encoded data is useful in cases where
base64 encoded data is concatenated / streamed.
I.e. where there are padding chars _within_ the stream.
In other cases padding is optional and can be inferred.
Note we continue to treat partial padding as invalid,
as that would be indicative of truncation.

* src/basenc.c (do_decode): Auto pad the end of the input.
* NEWS: Mention the change in behavior.
* tests/misc/base64.pl: Adjust to not fail for missing padding.
Addresses https://bugs.gnu.org/66265