Thanks to Alexander Ried for pointing out that if you have
tinc@.service template, systemd will provide a default slice containing
all instances of that template. So "systemctl start tinc" will still do
what we want it to do.
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.