Ensure socket-dir matches runtimedir on old systemd

Ensure socket-dir matches runtimedir on old systemd

This is mostly a hack for systemd version <240 so using virtual hosting
on Ubuntu Bionic and EL7 does not lead to surprises.

This Commit explicitly adds the `--socket-dir` to the invocation of the
unit. Any users who want to use their own runtimedirs should use a
drop-in unit overriding ExecStart. I believe this does not validate the
principle of least surprise for those using the PowerDNS provided
packages and virtual hosting.

Fixes #9485

(cherry picked from commit 7bbaae110abb53f43d0bf60c81efe3bb8630a925)