The users crate is no longer maintained. There is a maintained fork
called uzers, but we already have a dependency on nix, and that has the
functionality we need.
Unfortunately, clap_derive can't combine a value_parser of Option<T>
with an optional argument to get a merged Option<T> so we need to do the
check for the nop outside of the value parser.
Also adds some tests for --owner
Fixes: #1163