When updating the template for `chrony` to autogen a recent version, I removed `systemd` support because we Funtoo itself has no plans for `systemd` support and currently intentionally does not support it.  However!  Funtoo's tree is used downstream by Macaroni OS, and @geaaru has requested that we please preserve the `systemd` logic for downstream uses.

This issue is to add back what I removed:  A `systemd` USE flag and supporting service files, etc. for `net-misc/chrony`.