Interim interval

This code allows for interim accounting updates to be sent at an interval defined on the captive portal page, or by the server in the Access-Accept packet.

This is an untested patch, based on a working, tested patch for 2.0. I don't have a 2.1 test box set up for a couple of weeks yet.

See feature request 1492: http://redmine.pfsense.org/issues/1492