exec: Fixing ptrace'd mulit-threaded hang

Oleg your comment about kill being able to send signal was an important
dimension I had missed thank you.

This patchset just denies the case of SIGHAND between different
multi-threaded processes as I don't think anyone cares.  I can
fix that if anyone cares but I am not certain we actally do.

I have reworked the ptrace notification code so that we always
send notifications if we can but don't wait if it is a coredump
or an exec.  Which simpilifies the code nicely.

A few more tweaks are needed before a final version but I think
things are compelling.

 fs/exec.c                    |  23 ++-------
 include/linux/sched/signal.h |   1 +
 kernel/exit.c                |  20 ++++----
 kernel/fork.c                |  14 +++++-
 kernel/ptrace.c              |   4 ++
 kernel/signal.c              | 112 +++++++++++++++++++------------------------
 6 files changed, 78 insertions(+), 96 deletions(-)

Eric W. Biederman (5):
      ptrace: Don't wait in PTRACE_O_TRACEEXIT for exec or coredump
      sighand: Count each thread group once in sighand_struct
      clone: Disallown CLONE_THREAD with a shared sighand_struct
      exec: If possible don't wait for ptraced threads to be reaped
      signal: Don't allow accessing signal_struct by old threads after exec

Eric