Add https to the unit file DOC thing

Allow cmake to configure the systemd unit’s user and group

Only activate systemd’s watchdog if we are compiling with systemd

ref #3152

Generate systemd file from template and make installs it and the conf

The unit file Type=, as well as the executable path should now be correct
with a simple “make install”, whatever the install prefix and the DESTDIR
values are.

ref #3152

Do not specify an ExecStop= line in the unit file

Run the service with the nobody user

Provide an example unit file for systemd