This fixes a number of sys_execve() problems:

 - ptrace of thread groups over exec works again.

 - if the exec() is done in a non-leader thread then we must inherit the
   parent links properly - otherwise the shell will see an early
   child-exit notification.

 - if the exec()-ing thread is detached then make it use SIGCHLD like the
   leader thread.

 - wait for the leader thread to become TASK_ZOMBIE properly -
   wait_task_inactive() alone was not enough. This should be a rare
   codepath.

now sys_execve() from thread groups works as expected in every combination
i could test: standalone, from the leader thread, from one of the child
threads, ptraced, non-ptraced, SMP and UP.