dir auths only give Guard if they're giving Stable

This change allows us to simplify path selection for clients, and it
should have minimal effect in practice since >99% of Guards already have
the Stable flag. Implements ticket 18624.

changes/feature18624

@@ -0,0 +1,7 @@
+  o Minor features:
+    - Directory authorities now only give the Guard flag to a relay if
+      they are also giving it the Stable flag. This change allows us to
+      simplify path selection for clients, and it should have minimal
+      effect in practice since >99% of Guards already have the Stable
+      flag. Implements ticket 18624.
+

src/or/dirserv.c

@@ -2200,7 +2200,7 @@ set_routerstatus_from_routerinfo(routerstatus_t *rs,
 
   rs->is_valid = node->is_valid;
 
-  if (node->is_fast &&
+  if (node->is_fast && node->is_stable &&
       ((options->AuthDirGuardBWGuarantee &&
         routerbw_kb >= options->AuthDirGuardBWGuarantee/1000) ||
        routerbw_kb >= MIN(guard_bandwidth_including_exits_kb,