Simplify notification code in NetlinkHandler.

1. Factor most of the notification code out to a common function.
2. Use vasprintf instead of snprintf so we don't have to worry
   about clipping notifications due to fixed-size message
   buffers.

Bug: 9180552
Change-Id: Idde16ee6dd56d38dab866f0ea678b04d98b3048d
diff --git a/NetlinkHandler.h b/NetlinkHandler.h
index 0cbf339..c0396c0 100644
--- a/NetlinkHandler.h
+++ b/NetlinkHandler.h
@@ -33,6 +33,7 @@
 protected:
     virtual void onEvent(NetlinkEvent *evt);
 
+    void notify(int code, const char *format, ...);
     void notifyInterfaceAdded(const char *name);
     void notifyInterfaceRemoved(const char *name);
     void notifyInterfaceChanged(const char *name, bool isUp);