utmpupdate: Add a program for updating /var/run/utmp

To keep track of ongoing terminal sessions, we now have a sort-of
traditional /var/run/utmp file, like other Unix systems.
Unlike other Unix systems however, ours is of course JSON. :^)

The /bin/utmpupdate program is used to update the file, which is
not writable by regular user accounts. This helper program is
set-GID "utmp".

Userland/utmpupdate.cpp

@@ -0,0 +1,82 @@
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <LibCore/ArgsParser.h>
+#include <LibCore/File.h>
+#include <LibCore/DateTime.h>
+
+// utmpupdate -c /dev/pts/0
+// utmpupdate -d /dev/pts/0
+
+int main(int argc, char** argv)
+{
+    pid_t pid = 0;
+    bool flag_create = false;
+    bool flag_delete = false;
+    const char* tty_name = nullptr;
+    const char* from = nullptr;
+
+    Core::ArgsParser args_parser;
+    args_parser.add_option(flag_create, "Create entry", "create", 'c');
+    args_parser.add_option(flag_delete, "Delete entry", "delete", 'd');
+    args_parser.add_option(pid, "PID", "PID", 'p', "PID");
+    args_parser.add_option(from, "From", "from", 'f', "From");
+    args_parser.add_positional_argument(tty_name, "TTY name", "tty");
+
+    args_parser.parse(argc, argv);
+
+    if (flag_create && flag_delete) {
+        warn() << "-c and -d are mutually exclusive";
+        return 1;
+    }
+
+    dbg() << "Updating utmp from UID=" << getuid() << " GID=" << getgid() << " EGID=" << getegid() << " PID=" << pid;
+
+    auto file_or_error = Core::File::open("/var/run/utmp", Core::IODevice::ReadWrite);
+    if (file_or_error.is_error()) {
+        dbg() << "Error: " << file_or_error.error();
+        return 1;
+    }
+
+    auto& file = *file_or_error.value();
+
+    auto file_contents = file.read_all();
+    auto previous_json = JsonValue::from_string(file_contents);
+
+    JsonObject json;
+
+    if (!previous_json.has_value() || !previous_json.value().is_object()) {
+        dbg() << "Error: Could not parse JSON";
+    } else {
+        json = previous_json.value().as_object();
+    }
+
+    if (flag_create) {
+        JsonObject entry;
+        entry.set("pid", pid);
+        entry.set("uid", getuid());
+        entry.set("from", from);
+        entry.set("login_at", Core::DateTime::now().to_string());
+        json.set(tty_name, move(entry));
+    } else {
+        ASSERT(flag_delete);
+        dbg() << "Removing " << tty_name << " from utmp";
+        json.remove(tty_name);
+    }
+
+    if (!file.seek(0)) {
+        dbg() << "Seek failed";
+        return 1;
+    }
+
+    if (!file.truncate(0)) {
+        dbg() << "Truncation failed";
+        return 1;
+    }
+
+    if (!file.write(json.to_string())) {
+        dbg() << "Write failed";
+        return 1;
+    }
+
+    return 0;
+}