systemd: use unit name in PrivateTmp directories

Unit name is used whole in the directory name, so that the unit name
can be easily extracted from it, e.g. "/tmp/systemd-abcd.service-DEDBIF1".

https://bugzilla.redhat.com/show_bug.cgi?id=957439
catalog: remove links to non-existent wiki pages

AFAIK, we don't have even one page with message explanations.
If/when we add them, we can add links.

https://bugzilla.redhat.com/show_bug.cgi?id=1017161