pidof: Implemented -o (#123)

* pidof: Implemented `-o`

This commit implemented two usage of `-o` which are already implemented in GNU's implementation, but not in ours.

```shell
❯ cargo run pidof zsh
116473 116472 116401 116391

❯ cargo run pidof -o116473,116472 zsh
116401 116391

❯ cargo run pidof -o116473 -o116472 zsh
116401 116391
```

And the GNU's implementation:

```shell
❯ pidof zsh
116473 116472 116401 116391

❯ pidof -o116473,116472 zsh
116401 116391

❯ pidof -o116473 -o116472 zsh
116401 116391
```

* pidof: Add test for `-q` flag.

* pidof: Simplify tests.

src/uu/pidof/src/pidof.rs

@@ -137,6 +137,7 @@ pub fn uu_app() -> Command {
             Arg::new("o")
                 .short('o')
                 .help("Omit results with a given PID")
+                .value_delimiter(',')
                 .action(ArgAction::Append)
                 .value_parser(clap::value_parser!(usize))
                 .value_name("omitpid"),

tests/by-util/test_pidof.rs

@@ -33,3 +33,9 @@ fn test_no_program() {
 fn test_no_pid_found() {
     new_ucmd!().arg("NO_THIS_PROGRAM").fails().code_is(1);
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_quiet() {
+    new_ucmd!().arg("kthreadd").arg("-q").succeeds().no_output();
+}