mail-filter/opendkim: Improved OpenRC and systemd support

mail-filter/opendkim: Improved OpenRC and systemd support

Michael Orlitzky and I have been working on improved OpenRC support
for OpenDKIM. Our pull request has not yet been accepted upstream (not
even acknowledged, in fact). The multi-file patch adds our changes to
the existing Gentoo ebuild. Changes to the upstream build files
include:

* configure.ac: Added workaround to support @runstatedir@. On modern
  systems /run replaces /var/run, so using @runstatedir@ instead of
  @localstatedir@/run is preferable. Autoconf 2.70 will support
  @runstatedir@ , but it has not been released yet, so a workaround
  is required for version 2.61.

* contrib/systemd: Use @runstatedir@ in opendkim.service.in, enable
  full @VARIABLE@ substitution, fix inconsistencies in the systemd
  service file, add tmpfiles.d configuration file
  (https://bugs.gentoo.org/606978).

* opendkim/opendkim.conf.*.in: Use @runstatedir@ in PidFile paths,
  enable full @VARIABLE@ substitution.

Bug: https://bugs.gentoo.org/606978
Signed-off-by: Ralph Seichter <gentoo@seichter.de>
Package-Manager: Portage-2.3.62, Repoman-2.3.11
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>