Move atexit/on_exit data structures to task group; Now callbacks only occur...

Move atexit/on_exit data structures to task group; Now callbacks only occur when the final member of the task group exits

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5607 42af7a65-404d-4744-a932-0658087f49c3