Non-stop linux native.

* linux-nat.c (linux_test_for_tracefork): Block events while we're
	here.
	(get_pending_status): Implement non-stop mode.
	(linux_nat_detach): Stop threads before detaching.
	(linux_nat_resume): In non-stop mode, always resume only a single
	PTID.
	(linux_handle_extended_wait): On a clone event, in non-stop mode,
	add new lwp to GDB's thread table, and mark as running, executing
	and stopped appropriately.
	(linux_nat_filter_event): Don't assume there are other running
	threads when a thread exits.
	(linux_nat_wait): Mark the main thread as running and executing.
	In non-stop mode, don't stop all lwps.
	(linux_nat_kill): Stop lwps before killing them.
	(linux_nat_thread_alive): Use signal 0 to detect if a thread is
	alive.
	(send_sigint_callback): New.
	(linux_nat_stop): New.
	(linux_nat_add_target): Set to_stop to linux_nat_stop.

	* linux-nat.h (thread_db_attach_lwp): Declare.

	* linux-thread-db.c (thread_get_info_callback): Check for new
	threads if we have none.
	(thread_from_lwp, enable_thread_event): Set proc_handle.pid to the
	stopped lwp.  Check for new threads if we have none.
	(thread_db_attach_lwp): New.
	(thread_db_init): Set proc_handle.pid to inferior_ptid.
	(check_event): Set proc_handle.pid to the stopped lwp.
	(thread_db_find_new_threads): Set proc_handle.pid to any stopped
	lwp available, bail out if there is none.

	* linux-fork.c (linux_fork_killall): Use SIGKILL instead of
	PTRACE_KILL.

gdb/linux-fork.c

@@ -337,7 +337,9 @@ linux_fork_killall (void)
     {
       pid = PIDGET (fp->ptid);
       do {
-	ptrace (PT_KILL, pid, 0, 0);
+	/* Use SIGKILL instead of PTRACE_KILL because the former works even
+	   if the thread is running, while the later doesn't.  */
+	kill (pid, SIGKILL);
 	ret = waitpid (pid, &status, 0);
 	/* We might get a SIGCHLD instead of an exit status.  This is
 	 aggravated by the first kill above - a child has just