Systemd:  Fix server systemd unit

* Run the daemon with -D /run/bcfg2-server.pid.  If you don't run it
  with -D PID, it doesn't fork, and since the unit file is
  Type=forking, the service eventually times out and fails.
* Use the PIDFile unit option to give systemd a hint where the PID
  file is located.  This is recommended when using Type=forking by the
  systemd documentation.
* Use $BCFG2_SERVER_OPTIONS, which is defined in
  /etc/sysconfig/bcfg2-server, instead of $OPTIONS, which is not.