If no other units have dependencies on network-online.target or
syslog.target they might not get initialized resulting in a possibly
non-ideal startup order.
Closesstrongswan/strongswan#2279
Use strongswan-starter for the legacy unit and simply strongswan for the
modern one (strongswan-swanctl is configured as alias, which should
cause the installation of symlinks when the service is enabled via
systemctl).