Add mechanism to expose kernel variables to userspace via ProcFS.

Only booleans are supported at first. More types can be added easily.
Use this to add /proc/sys/wm_flash_flush which when enabled flashes pending
screen flush rects in yellow before they happen.

WindowServer/WSWindowManager.h

@@ -94,4 +94,6 @@ private:
     OwnPtr<Painter> m_front_painter;
 
     mutable Lock m_lock;
+
+    bool m_flash_flush { false };
 };