feature(uptime): add option --since

For this, I:
* moved from getops to clap
* remove the millisecond maths

tests/test_uptime.rs

@@ -1,3 +1,5 @@
+extern crate regex;
+use self::regex::Regex;
 use common::util::*;
 
 #[test]
@@ -12,3 +14,17 @@ fn test_uptime() {
     assert!(result.stdout.contains(" up "));
     // Don't check for users as it doesn't show in some CI
 }
+
+#[test]
+fn test_uptime_since() {
+    let scene = TestScenario::new(util_name!());
+
+    let result = scene.ucmd().arg("--since").succeeds();
+
+    println!("stdout = {}", result.stdout);
+    println!("stderr = {}", result.stderr);
+
+    assert!(result.success);
+    let re = Regex::new(r"\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}").unwrap();
+    assert!(re.is_match(&result.stdout.trim()));
+}