xargs: support -l option

Some of the failures in the Gnu findutils testsuite are due to xargs not supporting the lowercase `-l` option. It will be slightly tricky to implement this, as it requires the same argument parsing semantics as `-i` that the clap-rs library doesn't yet support (described in issue [594](https://github.com/uutils/findutils/issues/594)). But I think it should be possible to partially implement support for `-l` in the meantime. I'm willing to work on a PR to add `-l`.