o Minor bugfixes:
    - Allow tor to build circuits using a consensus with
      no exits. If the consensus has no exits (typical of
      a bootstrapping test network), allow tor to build
      circuits once enough descriptors have been
      downloaded.
      When there are no exits, we always have "enough"
      exit descriptors. (We treat the proportion of
      available exit descriptors as 100%.)
      This assists in bootstrapping a testing Tor
      network.
      Fixes bug 13718.
      Makes bug 13161's TestingDirAuthVoteExit
      non-essential.
      (But still useful for speeding up a bootstrap.)