cksum: Use clap's usize parser and default_value("0")

[oech3]

Currently, cksum's length is managed by Option:

coreutils/src/uu/checksum_common/src/cli.rs

Lines 74 to 82 in 0f38c81

	fn with_length(self) -> Self {
	self.arg(
	Arg::new(options::LENGTH)
	.long(options::LENGTH)
	.short('l')
	.help(translate!("ck-common-help-length"))
	.action(ArgAction::Set),
	)
	}

But cksum -l 0 is supported for alias for default value. So Some() is unnecessary if clap provided defaukt value with usize parser.