This adds non-stop support for linux native.

The changes are:

- ptracing a running thread doesn't work.

 This implies that, we must ensure that the proc_services
 usage in linux-thread-db.c talks to a pid of a stopped lwp.

 Checking if a thread is alive with ptrace doesn't work
 for running threads.  Worse, ptrace errors out claiming
 the thread doesn't exits.

- We must not stop all threads, obviously.

- We must mark threads as running if we're resuming
 them behind the core's back.

- Implement target_stop_ptid to interrupt only one thread

-- 
Pedro Alves