users: support OpenBS using utmp

OpenBSD uses the original utmp file format so the general `users` does not
work there. By using the `utmp-classic` lib, we cna process the current users
on the system from the /var/run/utmp and show the list.

fixes #5665

tests/by-util/test_users.rs

@@ -31,3 +31,12 @@ fn test_users_check_name() {
 
     new_ucmd!().succeeds().stdout_is(&expected);
 }
+
+#[test]
+#[cfg(target_os = "openbsd")]
+fn test_users_check_name_openbsd() {
+    new_ucmd!()
+        .args(&["openbsd_utmp"])
+        .run()
+        .stdout_contains("test");
+}