systemctl: report accounted network traffic in "systemctl status"

systemctl: report accounted network traffic in "systemctl status"

This hooks up the eposed D-Bus values and displays them like this:

-bash-4.3# systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/etc/systemd/system/httpd.service; enabled; vendor preset: disabled)
   Active: active (running) since Fri 2016-11-11 20:10:36 CET; 1min 29s ago
 Main PID: 33 (httpd)
   Status: "Total requests: 22514; Idle/Busy workers 92/7;Requests/sec: 259; Bytes served/sec:  87KB/sec"
  Network: 15.8M in, 51.1M out
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   CGroup: /system.slice/httpd.service
           ├─ 33 /usr/sbin/httpd -DFOREGROUND
           ├─ 37 /usr/sbin/httpd -DFOREGROUND
           ├─112 /usr/sbin/httpd -DFOREGROUND
           └─119 /usr/sbin/httpd -DFOREGROUND