We are going to remove the --syslog option to log everything into
syslog. However, netlog messages received from netlog services should
still go into syslog. So use syslog() for that explicitly.

We now run clusterd as a systemd service. The features to daemonize,
redirect logging, restart and kill previous daemons are all not needed.
Additionally, the restart is buggy, because
Donald::Tools::kill_previous_server doesn't kill the parent - assuming
this is an rc.d script. However, in `--foregound` mode, the previous
daemon is in fact the parent.

- Remove unused features and leave job control and logging to systemd.
- Log into journal instead of into syslog.
- Remove "Description=" from the unit, because its easier to identify
  things, when the name used in messages is the same as the unit name
  and the unit file name.