Split most of dirserv.c into several new modules

In dirauth:
  * bwauth.c reads and uses bandwidth files
  * guardfraction.c reads and uses the guardfraction file
  * reachability.c tests relay reachability
  * recommend_pkg.c handles the recommended-packages lines.
  * recv_descs.c handles fingerprint files and processing incoming
    routerinfos that relays upload to us
  * voteflag.c computes flag thresholds and sets those thresholds on
    routerstatuses when computing votes

In control:
  * fmt_serverstatus.c generates the ancient "v1 server status"
    format that controllers expect.

In nodelist:
  * routerstatus_fmt.c formats routerstatus entries for a consensus,
    a vote, or for the controller.