This patch makes it possible to identify bots by inspecting HTTP request
headers.  A CSV file, specified by BLACKLISTED_REQUEST_HEADERS_FILE,
contains mappings from request header to a regular expression of the
header's value, e.g.:

  Accept-Language,[Kk]lingon
  User-Agent,Spa+ce
  ...

Once a regular expression matches a client's request, we probably caught
a bot.  This patch also makes it possible to respond to bot requests
with a decoy bridge, e.g., to study what the owners of the bot intend to
do with the bridge.  Decoy bridges are configured in the CSV file
DECOY_BRIDGES_FILE.  The file maps a transport type and its IP address
version to bridge lines, e.g.:

  vanillav4,1.2.3.4:1234 FINGERPRINT
  obfs4v4,obfs4 1.2.3.4:1234 FINGERPRINT ARGS
  ...

This fixes <https://bugs.torproject.org/31252>