Userland: Add a simple 'w' program that shows current terminal sessions

This fetches information from /var/run/utmp and shows the interactive
sessions in a little list. More things can be added here to make it
more interesting. :^)

Userland/w.cpp

@@ -0,0 +1,62 @@
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <LibCore/File.h>
+#include <pwd.h>
+#include <stdio.h>
+
+int main()
+{
+    if (pledge("stdio rpath", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+
+    if (unveil("/etc/passwd", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    if (unveil("/var/run/utmp", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    unveil(nullptr, nullptr);
+
+    auto file_or_error = Core::File::open("/var/run/utmp", Core::IODevice::ReadOnly);
+    if (file_or_error.is_error()) {
+        warn() << "Error: " << file_or_error.error();
+        return 1;
+    }
+    auto& file = *file_or_error.value();
+    auto json = JsonValue::from_string(file.read_all());
+    if (!json.has_value() || !json.value().is_object()) {
+        warn() << "Error: Could not parse /var/run/utmp";
+        return 1;
+    }
+
+    printf("%-10s %-12s %-16s %-16s\n",
+        "USER", "TTY", "FROM", "LOGIN@");
+    json.value().as_object().for_each_member([&](auto& tty, auto& value) {
+        const JsonObject& entry = value.as_object();
+        auto uid = entry.get("uid").to_u32();
+        auto pid = entry.get("pid").to_i32();
+        (void)pid;
+        auto from = entry.get("from").to_string();
+        auto login_at = entry.get("login_at").to_string();
+
+        auto* pw = getpwuid(uid);
+        String username;
+        if (pw)
+            username = pw->pw_name;
+        else
+            username = String::number(uid);
+
+        printf("%-10s %-12s %-16s %-16s\n",
+            username.characters(),
+            tty.characters(),
+            from.characters(),
+            login_at.characters());
+    });
+    return 0;
+}