Implement an NX interface to block flush message queues in multi-user mode. ...

Implement an NX interface to block flush message queues in multi-user mode.  This is necessary to prevent stale window handles when a window is closed

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@4745 42af7a65-404d-4744-a932-0658087f49c3