Make begindir advertise checks consistent with DirPort checks
This ticket makes sure the checks that Tor does when advertising begindir support are similar to the checks it does when advertising the DirPort.
- bridges should advertise begindir support
- authorities should always advertise begindir
- we should never advertise begindir if the network is disabled
- we should never advertise begindir if we don't have an ORPort (redundant, as we don't post descriptors without an ORPort)
- relays should handle AccountingMax like they do for DirPort