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.

Change-Id: I2f1e807786e484774e5a1a97c297532d86f99265
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>