pidof: don't require program name

[cakebaker]

Currently, we require at least one program name and if none is provided, a clap error message is shown. This makes the following code snippet in uumain unreachable:

    if arg_program_name.is_none() {
        uucore::error::set_exit_code(1);
        return Ok(());
    };

This PR no longer requires a program name and thus makes the snippet above reachable. It also corresponds with the behavior of the original pidof, which doesn't show an error message if called without any args.

The PR also removes the #[cfg(target_os = "linux")] of the test as it is not necessary.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (92d674b) to head (5ca17df).
Report is 8 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #359   +/-   ##
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