Add PidFile support to Arti for systemd and init.d

Summary

In order to correctly support stopping and starting arti.service or an arti init.d script, PidFile support is required from Arti (similar to what c-tor has).

What is the expected behavior?

Systemd needs support for "PIDFile=" in order to correctly determine if the arti.service is running when combined with init.d support.