basenc: ignore case with "--base16 --decode"

[cakebaker]

Currently, an error is shown if the input provided to basenc --base16 --decode contains lowercase characters. This PR fixes this issue and allows lowercase characters. It also makes the tests b16_9 and b16_10 in basenc.pl pass.

// before change
$ printf "48656c6c6f2c20576f726c6421" | cargo run basenc --base16 -d
basenc: error: invalid input

// after change
$ printf "48656c6c6f2c20576f726c6421" | cargo run basenc --base16 -d
Hello, World!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)