NAS accounting

This code allows pfSense to send Accounting-On and Accounting-Off RADIUS packets when the captive portal service changes state:
Accounting-Off when pfSense is shutdown or CP is disabled
Accounting-On when pfSense is booting

The main advantage of this is to allow the remote RADIUS server to know when the NAS might have been unexpectedly rebooted without stopping its active sessions, and take appropriate action.

See feature request 2143: http://redmine.pfsense.org/issues/2143