Move waitpid() data structures to task group; The caller of waitpid() is now...

Move waitpid() data structures to task group; The caller of waitpid() is now only awakened when the final thread of the task group exits

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5608 42af7a65-404d-4744-a932-0658087f49c3