restore: unset dumpable flag for zombies (v2)

We have already tried to prevent generating core files for zombies:

commit 6da52216
Author: Andrey Vagin <avagin@openvz.org>
Date:   Fri Jul 12 18:14:23 2013 +0400

    restore: set the zero limit for RLIMIT_CORE

But it doesn't work if a core file is sent into a pipe.
This functionality is used by the abrt daemon for example.

This patch uses more direct way, it unsets the dumpable flag with help
of PR_SET_DUMPABLE.

v2: remove the previous hack

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>