man: document all pager variables for systemctl and systemd

man: document all pager variables for systemctl and systemd

In those two pages, we need to include individual entries with xi:include to
merge the list less-variables.xml with the other entries, which is obviously
error prone. All variables are supported in both tools so add them.

(cherry picked from commit 3d58bfc01c9283eded77736990ea09b0ee4f6e23)