do not die trying to chmod() a hardlink

Signed-off-by: Michal Toman <mtoman@redhat.com>
set_atomic: allow to append to non-existing file

Signed-off-by: Michal Toman <mtoman@redhat.com>
only hardlink unpacked vmcores

Signed-off-by: Michal Toman <mtoman@redhat.com>