ls: make --block-size and --human-readable/--si override each other

[cakebaker]

This PR makes --block-size and --human-readable/--si override each other to match the behavior of GNU ls.

[tertsdiepraam]

Excellent! A related problem is:

> ls --human-readable --human-readable
error: the argument '--human-readable' cannot be used multiple times

Usage: target/debug/coreutils ls [OPTION]... [FILE]...

I think it should use https://docs.rs/clap/latest/clap/struct.Command.html#method.args_override_self

But feel free to merge without that.

[cakebaker]

Good point, I enabled it in the latest push.

[tertsdiepraam]

Great! :)