More work towards making bridge users able to connect via bridges:

More work towards making bridge users able to connect via bridges:
  - demand options->Bridges and options->TunnelDirConns if
    options->UseBridges is set.
  - after directory fetches, accept descriptors that aren't referenced by
    our networkstatuses, *if* they're for a configured bridge.
  - delay directory fetching until we have at least one bridge descriptor.
  - learn how to build a one-hop circuit when we have neither routerinfo
    nor routerstatus for our destination.
  - teach directory connections how to pick a bridge as the destination
    directory when doing non-anonymous fetches.
  - tolerate directory commands for which the dir_port is 0.
  - remember descriptors when the requested_resource was "authority",
    rather than just ignoring them.
  - put bridges on our entry_guards list once we have a descriptor for them.
    When UseBridges is set, only pick entry guards that are bridges. Else
    vice versa.

svn:r10571