Merge remote-tracking branch 'nickm/prop110_v2'

  o Major features:

    - Now that Tor 0.2.0.x is completely deprecated, we can enable the

      final part of "Proposal 110: Avoiding infinite length circuits"

      by refusing all circuit-extend requests that do not appear in a

      "relay_early" cell. This change helps Tor to resist a class of

      denial-of-service attacks by limiting the maximum circuit length.

        connection_mark_and_flush(TO_CONN(conn));

      }

      return 0;

    case RELAY_COMMAND_EXTEND:

    case RELAY_COMMAND_EXTEND: {

      static uint64_t total_n_extend=0, total_nonearly=0;

      total_n_extend++;

      if (conn) {

        log_fn(LOG_PROTOCOL_WARN, domain,

               "'extend' cell received for non-zero stream. Dropping.");

        return 0;

      }

      if (cell->command != CELL_RELAY_EARLY &&

          !networkstatus_get_param(NULL,"AllowNonearlyExtend",0,0,1)) {

#define EARLY_WARNING_INTERVAL 900

        static ratelim_t early_warning_limit =

          RATELIM_INIT(EARLY_WARNING_INTERVAL);

        char *m;

        if (cell->command == CELL_RELAY) {

          ++total_nonearly;

          if ((m = rate_limit_log(&early_warning_limit, approx_time()))) {

            /* XXXX make this a protocol_warn once we're happier with it*/

            double percentage = ((double)total_nonearly)/total_n_extend;

            percentage *= 100;

            log_fn(LOG_WARN, domain, "EXTEND cell received, "

                   "but not via RELAY_EARLY. Dropping.%s", m);

            log_fn(LOG_WARN, domain, "  (We have dropped %.02f%% of all "

                   "EXTEND cells for this reason)", percentage);

            tor_free(m);

          }

        } else {

          log_fn(LOG_WARN, domain,

                 "EXTEND cell received, in a cell with type %d! Dropping.",

                 cell->command);

        }

        return 0;

      }

      return circuit_extend(cell, circ);

    }

    case RELAY_COMMAND_EXTENDED:

      if (!layer_hint) {

        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,