Split systemctl and tmpfiles into a separate package

The backstory is that %{?systemd_requires} in all kinds of .spec files
pulls in systemd, which is not always desirable. Upstream has come up
with some solution where they make use of a rpm tag (available in
4.10+) that is only about ordering - cf
https://github.com/systemd/systemd/commit/2424b6bd716f0c1c3bf3406b1fd1a16ba1b6a556
. The commit explicitly states though, that "installing systemd
afterwards [...], does not result in the same outcome."

With this proposed change to our systemd.spec, we could have the cake
and eat it too:

* symlinks in /etc/systemd can be created at install time, permitting
the administrator to install/use systemd as init system in a container
at a later date (enabling "same outcome")

* mandatory tmp directories will be created at install time already,
so the administrator does not have to run `tmpfiles --root=/mycontainer`
before launching the (systemd-less) container.

* the only protruding dependency of systemctl.rpm is libgcrypt20,
i.e. comparatively light.

* the OrderWithRequires feature can be done independently

Sounds like a plan? @mpluskal too #boo980389