add preliminary RADIUS MIB

add preliminary RADIUS MIB

./scripts/bin/radict -F struct -M -p radius mib-2.radiusAuthServ

which doesn't have the client table, but it's a start.  And the
resulting structure isn't too bad.

Todo:

* have callbacks for specific fields.  e.g. uptime.
  * Those callbacks should be defined globally, and passed a uctx.
  * the callback can then update the field before it's copied out
    to the caller