Add a type and version header to the fallback directory mirror file

  o Minor features (fallback directory mirrors):

    - Add a type and version header to the fallback directory mirror file.

      This helps external parsers like stem and Relay Search.

      Implements ticket 24725.

## Top-Level Configuration

# We use semantic versioning: https://semver.org

# In particular:

# * major changes include removing a mandatory field, or anything else that

#   would break an appropriately tolerant parser,

# * minor changes include adding a field,

# * patch changes include changing header comments or other unstructured

#   content

FALLBACK_FORMAT_VERSION = '2.0.0'

# Output all candidate fallbacks, or only output selected fallbacks?

OUTPUT_CANDIDATES = False

  """ Fetches required onionoo documents and evaluates the

      fallback directory criteria for each of the relays """

  print "/* type=fallback */"

  print ("/* version={} */"

         .format(cleanse_c_multiline_comment(FALLBACK_FORMAT_VERSION)))

  logging.warning('Downloading and parsing Onionoo data. ' +

                  'This may take some time.')

  # find relays that could be fallbacks