Warn when two hs use the same directory

  o Minor bugfixes:

    - Warn when the user configures two HiddenServiceDir lines that point

      to the same directory. Bugfix on 0.0.6 (the version introducing

      HiddenServiceDir); fixes bug 3289.

    log_warn(LD_CONFIG, "Hidden service with no ports configured; ignoring.");

    rend_service_free(service);

  } else {

    int dupe = 0;

    /* XXX This duplicate check has two problems:

     *

     * a) It's O(n^2), but the same comment from the bottom of

     *    rend_config_services() should apply.

     *

     * b) We only compare directory paths as strings, so we can't

     *    detect two distinct paths that specify the same directory

     *    (which can arise from symlinks, case-insensitivity, bind

     *    mounts, etc.).

     *

     * It also can't detect that two separate Tor instances are trying

     * to use the same HiddenServiceDir; for that, we would need a

     * lock file.  But this is enough to detect a simple mistake that

     * at least one person has actually made.

     */

    SMARTLIST_FOREACH(rend_service_list, rend_service_t*, ptr,

                      dupe = dupe ||

                             !strcmp(ptr->directory, service->directory));

    if (dupe) {

      log_warn(LD_REND, "Another hidden service is already configured for "

               "directory %s, ignoring.", service->directory);

      rend_service_free(service);

      return;

    }

    smartlist_add(rend_service_list, service);

    log_debug(LD_REND,"Configuring service with directory \"%s\"",

              service->directory);