prctl: add PR_[GS]ET_PDEATHSIG_PROC

PR_SET_PDEATHSIG sets a parent death signal that the calling process will
get when its parent thread dies, even when the result of getppid() doesn't
change because the calling process is reparented to a different thread in
the same parent process.  When managing multiple processes, a
process-based parent death signal is much more useful.  E.g., to avoid
stray child processes.

PR_SET_PDEATHSIG_PROC sets a process-based death signal.  Unlike
PR_SET_PDEATHSIG, this is inherited across fork to allow killing a whole
subtree without race conditions.

This can be used for sandboxing when combined with a seccomp filter.

There have been previous attempts to support this by changing the behavior
of PR_SET_PDEATHSIG.  However, that would break existing applications.
See https://marc.info/?l=linux-kernel&m=117621804801689 and
https://bugzilla.kernel.org/show_bug.cgi?id=43300

Link: http://lkml.kernel.org/r/20170929123058.48924-1-j@bitron.ch
Signed-off-by: Jürg Billeter <j@bitron.ch>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Filipe Brandenburger <filbranden@google.com>
Cc: David Wilcox <davidvsthegiant@gmail.com>
Cc: "Adam H . Peterson" <alphaetapi@hotmail.com>
Cc: <hansecke@gmail.com>
Cc: <linux-api@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

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