uptime: Bring output closer to Linux/BSDs, and add -p flag

The -p flag is equivalent to the previous behavior: outputting the
uptime in a human-readable form.

We don't seem to expose the number of online users or the load averages,
so those sections are missing from the output compared to those OSes.

Base/usr/share/man/man1/uptime.md

@@ -8,9 +8,23 @@ uptime - Tell how long the system has been running
 $ uptime
 ```
 
+## Description
+
+`uptime` outputs information about the system, in a single line, to STDOUT.
+This information includes when the system came online and how long it has been up.
+
+## Options
+
+* `-p`, `--pretty`: Output only the uptime, in human-readable format.
+
 ## Examples
 
 ```sh
 $ uptime
+2024-01-24 06:23:27 up 4:20:00
+```
+
+```sh
+$ uptime -p
 Up 2 minutes, 20 seconds
 ```

Userland/Utilities/uptime.cpp

@@ -1,19 +1,28 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <[email protected]>
  * Copyright (c) 2022, Karol Kosek <[email protected]>
+ * Copyright (c) 2024, Sam Atkins <[email protected]>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <AK/NumberFormat.h>
+#include <LibCore/ArgsParser.h>
+#include <LibCore/DateTime.h>
 #include <LibCore/File.h>
 #include <LibCore/System.h>
 #include <LibMain/Main.h>
 
-ErrorOr<int> serenity_main(Main::Arguments)
+ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
     TRY(Core::System::pledge("stdio rpath"));
 
+    bool pretty_output = false;
+
+    Core::ArgsParser args_parser;
+    args_parser.add_option(pretty_output, "Output only the uptime, in human-readable format", "pretty", 'p');
+    args_parser.parse(arguments);
+
     auto file = TRY(Core::File::open("/sys/kernel/uptime"sv, Core::File::OpenMode::Read));
 
     TRY(Core::System::pledge("stdio"));
@@ -25,6 +34,14 @@ ErrorOr<int> serenity_main(Main::Arguments)
         return Error::from_string_literal("Couldn't convert to number");
     auto seconds = maybe_seconds.release_value();
 
-    outln("Up {}", human_readable_time(seconds));
+    if (pretty_output) {
+        outln("Up {}", human_readable_time(seconds));
+    } else {
+        auto current_time = TRY(Core::DateTime::now().to_string());
+        // FIXME: To match Linux and the BSDs, we should also include the number of current users,
+        //        and some load averages, but these don't seem to be available yet.
+        outln("{} up {}", current_time, human_readable_digital_time(seconds));
+    }
+
     return 0;
 }