base32/base64/basenc: implement and test proper flag parsing

[BenWiederhake]

This PR fixes several issues:

• Basic functionality of basenc was not tested – it already works, yes, but let's make sure it doesn't regress.
• basenc should not decide according to some arbitrary built-in priority list (ENCODINGS), but rather let the last encoding on the command-line override any earlier ones. (This fixes a GNU behavior bug.)
• Permit repeated flags on the command-line; i.e. the flags -d, -i, and -w. (This fixes a GNU behavior bug.)

This fixes #5998 for the special case of base32, base64, and basenc.