tmpfiles: split tmp.conf out

tmpfiles: split tmp.conf out

tmp.conf was dealing with 2 different kind of paths: one dealing with general
temporary paths such as /var/tmp and /tmp and the other one dealing with
temporary directories owned by systemd.

If for example a user wants to adjust the age argument of the general paths
only, he had to overload the whole file which is cumbersome and error prone
since any future changes in tmp.conf shipped by systemd will be lost.

So this patch splits out tmp.conf so the systemd directories are dealt
separately in a dedicated conf file. It's named "systemd-tmp.conf" based on the
naming recommendation made in tmpfiles.d man page.

In practice it shouldn't cause any regression since it's very unlikely that
users override paths owned by systemd.