Implement event loop timers.

GObjects can now register a timer with the GEventLoop. This will eventually
cause GTimerEvents to be dispatched to the GObject.

This needed a few supporting changes in the kernel:

- The PIT now ticks 1000 times/sec.
- select() now supports an arbitrary timeout.
- gettimeofday() now returns something in the tv_usec field.

With these changes, the clock window in guitest2 finally ticks on its own.

Kernel/Process.h

@@ -331,6 +331,8 @@ private:
     int m_blocked_fd { -1 };
     Vector<int> m_select_read_fds;
     Vector<int> m_select_write_fds;
+    timeval m_select_timeout;
+    bool m_select_has_timeout { false };
     size_t m_max_open_file_descriptors { 16 };
     SignalActionData m_signal_action_data[32];
     dword m_pending_signals { 0 };