openvpn: make systemd dependency optional

systemd is a fairly large dependency, and it doesn't appear to
be necessary in all circumstances - e.g. when openvpn is
not run as a systemd service (as is usually the case when it is
run in a Docker container).

This change makes the dependency on systemd optional, controlled
by a new argument `useSystemd`. The default behaviour remains
the same as it was before this change: enabled only on Linux systems.

For me, this change reduces the size of my container image (dominated
by the closure of openvpn) from about 110 MB to 45 MB.

Version 2: rename argument to `useSystemd` (was `systemdSupport`), and
rebase onto master

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