Optionally install systemd service files.

If --with-systemd is given when running the configure script, two
systemd service files will be installed. There is a template
tinc@.service, which can be used to control individual instances of
tinc. For example:

systemctl enable tinc@foo

Will create an instance for tinc with netname foo. There is also a
tinc.service, which can be used to start and stop all instances at once.