RedHat: Use systemd unit files on RHEL >= 7

Handle rhel >= 7 like fedora >= 15 when deciding whether
to package systemd unit files or sysvinit scripts in the
rpm spec file.

Reviewed-on: http://gerrit.openafs.org/10631
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
(cherry picked from commit 534f802ac900df4704f4e8397eca0aeccf169023)

Change-Id: I98f5844b30a6baa85061864f58f64be80f2c7b62
Reviewed-on: http://gerrit.openafs.org/10767
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>