who: move from getopts to clap

[ycd]

Code is in:
https://github.com/uutils/coreutils/blob/master/src/uu/who/src/who.rs

Tests:
https://github.com/uutils/coreutils/blob/master/tests/by-util/test_who.rs

As example:
9081e12

[jhscheer]

I'm working on this.