ptrace: Stop supporting SIGKILL for PTRACE_EVENT_EXIT

Recently I had a conversation where it was pointed out to me that
SIGKILL sent to a tracee stropped in PTRACE_EVENT_EXIT is quite
difficult for a tracer to handle.

Keeping SIGKILL working after the process has been killed is pain
from an implementation point of view.

So since the debuggers don't want this behavior let's see if we can
remove this wart for the userspace API

If a regression is detected it should only need to be the last change
that is the reverted.  The other two are just general cleanups that
make the last patch simpler.

Eric W. Biederman (3):
      signal: Ensure SIGNAL_GROUP_EXIT gets set in do_group_exit
      signal: Guarantee that SIGNAL_GROUP_EXIT is set on process exit
      signal: Drop signals received after a fatal signal has been processed

 fs/coredump.c                |  2 +-
 include/linux/sched/signal.h |  1 +
 kernel/exit.c                | 20 +++++++++++++++++++-
 kernel/fork.c                |  2 ++
 kernel/signal.c              |  3 ++-
 5 files changed, 25 insertions(+), 3 deletions(-)
-----BEGIN PGP SIGNATURE-----
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=gX4y
-----END PGP SIGNATURE-----