nohup: move from getopts to clap (#1871)

- changed some error return codes to match GNU implementation
- changed warning/error messages to match GNU nohup
- replaced getopts dependency with clap
- added a test

tests/by-util/test_nohup.rs

@@ -1 +1,19 @@
-// ToDO: add tests
+use crate::common::util::*;
+use std::thread::sleep;
+
+// General observation: nohup.out will not be created in tests run by cargo test
+// because stdin/stdout is not attached to a TTY.
+// All that can be tested is the side-effects.
+
+#[test]
+#[cfg(any(target_os = "linux", target_os = "freebsd", target_vendor = "apple"))]
+fn test_nohup_multiple_args_and_flags() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    ucmd.args(&["touch", "-t", "1006161200", "file1", "file2"])
+        .succeeds();
+    sleep(std::time::Duration::from_millis(10));
+
+    assert!(at.file_exists("file1"));
+    assert!(at.file_exists("file2"));
+}