cups: restore correct on-demand startup and listening behaviour

The CUPS systemd setup received a change of setup.

The state after this update as intended:

The CUPS Server is not default started, but is started on-demand on first
access by any printer tool like "lp" or "kprinter" or any UI print dialog.

The "network browsing" for network printers however will only start once the
server runs, so it will take a bit after the on-demand startup for auto discovered
printers to show up.

If you want to enable the CUPS server on system start, as "root" user run once:

systemctl enable cups.service

Fixed bugs
bnc#857372
systemd socket activation by default either works insecure (CVE-2012-6094 and VUL-0: listening everywhere) or it does not work
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