Support 'none' in torrc for HiddenServiceExportCircuitID.

See: https://bugs.torproject.org/4700

doc/tor.1.txt

@@ -2838,7 +2838,8 @@ The following options are used to configure a hidden service.
 [[HiddenServiceExportCircuitID]] **HiddenServiceExportCircuitID** __protocol__::
    The onion service will use the given protocol to expose the global circuit
    identifier of each inbound client circuit via the selected protocol. The only
-   protocol supported right now \'haproxy\'. This option is only for v3 services.
+   protocol supported right now \'haproxy\'. This option is only for v3
+   services. (Default: none)
 
 [[HiddenServiceMaxStreams]] **HiddenServiceMaxStreams** __N__::
    The maximum number of simultaneous streams (connections) per rendezvous

src/feature/hs/hs_config.c

@@ -161,8 +161,11 @@ helper_parse_circuit_id_protocol(const char *key, const char *value, int *ok)
   if (! strcasecmp(value, "haproxy")) {
     *ok = 1;
     ret = HS_CIRCUIT_ID_PROTOCOL_HAPROXY;
+  } else if (! strcasecmp(value, "none")) {
+    *ok = 1;
+    ret = HS_CIRCUIT_ID_PROTOCOL_NONE;
   } else {
-    log_warn(LD_CONFIG, "%s must be 'haproxy'.", key);
+    log_warn(LD_CONFIG, "%s must be 'haproxy' or 'none'.", key);
     goto err;
   }