debian/control: Bump Standards-Version to 3.9.4.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
dh-exec-install-rename: Preserve permissions

When copying files, use File::Copy::cp(), because that preserves
permissions. Added a test, and an appropriate perl-modules dependency.

Thanks to Axel Beckert for the bugreport and assistance.

Fixes: #706939
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>