Allow RADIUS accounting to be reversed

According to [RFC 2866](http://tools.ietf.org/html/rfc2866#page-14), Acct-Input-Octets and Acct-Output-Octets are supposed to be determined from the point of view of the NAS, so input represents the count of data uploaded from the client, and output represents data downloaded to the client. But most RADIUS vendors reverse the meaning so that input is download and output is upload. pfSense follows the RFC, but it would be nice if there was an option to behave like the majority of other NAS devices out there.

More details here: http://coova.org/CoovaChilli/RADIUS