Add on_exit(); Re-order some logic in the task shutdown sequence. Sometimes...

Add on_exit(); Re-order some logic in the task shutdown sequence.  Sometimes some complex logic needs to execute when closing file descriptors and this needs to happen early while the task is still healthy

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@4354 42af7a65-404d-4744-a932-0658087f49c3