Unclear language speicification make it very hard to define and implement the class java.lang.ThreadGroup()

1. activeCount(), the API indicates it should return the count of active threads
  in this thread group. But there is no definition active. Actually, the thread
  can be started or stopped, or just exits any time. There is no semantic mean
  for that.

2. There are 8 methods in the ThreadGroup API have never been implemented. They
   are threadsCount(), allThreadsCount(), groupsCount(), allGroupsCount(),
   threads(), allThreads(), groups(), allGroups()
   Is there any real need for these methods? What is semantics for the methods?
   Is it safe to do so? Since we need to take snapshot of thread group, which
   is likely to prevent a high priority thread to start or stop.

3. Since most os does not provide equivalent concept, it is very hard to map
   the definition to os primitives.

We should make the api more clear before we provide the implementation. This bug
is response for bug 4083644.