systemd: change the WantedBy target
    
    Previously with configuration WantedBy=network.target it could happen
    that after enabling let's say zebra and bgpd and restarting the server
    services won't be started. In case when no service pulls in
    network.target quagga daemons won't start, because they are enabled
    under network.target. This might be the case when only zebra is used
    for configuring interfaces and other services like NM and initscripts
    are disabled.
    
    This commit also removes ordering dependency on syslog.target, since
    this is redundant these days, because systemd-journald is early boot
    service.