Service intro circuits are now 4-hop to avoid linkability by intro

Now the circuits look like this:

	 client rend:   C -> G -> L2 -> Rend
	 client intro:  C -> G -> L2 -> M -> Intro
	 client hsdir:  C -> G -> L2 -> M -> HSDir
	 service rend:  C -> G -> L2 -> M -> Rend
	 service intro: C -> G -> L2 -> M -> Intro
	 service hsdir: C -> G -> L2 -> M -> HSDir

src/core/or/circuitbuild.c

@@ -1394,16 +1394,14 @@ route_len_for_purpose(uint8_t purpose, extend_info_t *exit_ei)
     return routelen;
 
   switch (purpose) {
-    /* These two purposes connect to a router that we chose, so
-     * DEFAULT_ROUTE_LEN is safe. */
-  case CIRCUIT_PURPOSE_S_ESTABLISH_INTRO:
-    /* hidden service connecting to introduction point */
+    /* These purposes connect to a router that we chose, so DEFAULT_ROUTE_LEN
+     * is safe: */
   case CIRCUIT_PURPOSE_TESTING:
     /* router reachability testing */
     known_purpose = 1;
     break;
 
-    /* These three purposes connect to a router that someone else
+    /* These purposes connect to a router that someone else
      * might have chosen, so add an extra hop to protect anonymity. */
   case CIRCUIT_PURPOSE_C_GENERAL:
   case CIRCUIT_PURPOSE_C_HSDIR_GET:
@@ -1413,6 +1411,9 @@ route_len_for_purpose(uint8_t purpose, extend_info_t *exit_ei)
     /* client connecting to introduction point */
   case CIRCUIT_PURPOSE_S_CONNECT_REND:
     /* hidden service connecting to rendezvous point */
+  case CIRCUIT_PURPOSE_S_ESTABLISH_INTRO:
+    /* hidden service connecting to intro point. In this case we want an extra
+       hop to avoid linkability attacks by the introduction point. */
     known_purpose = 1;
     routelen++;
     break;

src/test/test_circuitbuild.c

@@ -113,7 +113,7 @@ test_new_route_len_safe_exit(void *arg)
   /* hidden service connecting to introduction point */
   r = new_route_len(CIRCUIT_PURPOSE_S_ESTABLISH_INTRO, &dummy_ei,
                     &dummy_nodes);
-  tt_int_op(DEFAULT_ROUTE_LEN, OP_EQ, r);
+  tt_int_op(DEFAULT_ROUTE_LEN+1, OP_EQ, r);
 
   /* router testing its own reachability */
   r = new_route_len(CIRCUIT_PURPOSE_TESTING, &dummy_ei, &dummy_nodes);