271: Algorithm tweak to allow multiple entry guards.

Previously, we had NumEntryGuards kind of hardwired to 1.  Now we
have the code (but not the configuarability) to choose randomly from
among the first N primary guards that would work, where N defaults
to 1.

Part of 20831 support for making NumEntryGuards work again.

src/or/entrynodes.c

@@ -1795,6 +1795,9 @@ select_entry_guard_for_circuit(guard_selection_t *gs,
   if (!gs->primary_guards_up_to_date)
     entry_guards_update_primary(gs);
 
+  int num_entry_guards = 1;
+  smartlist_t *usable_primary_guards = smartlist_new();
+
   /* "If any entry in PRIMARY_GUARDS has {is_reachable} status of
       <maybe> or <yes>, return the first such guard." */
   SMARTLIST_FOREACH_BEGIN(gs->primary_entry_guards, entry_guard_t *, guard) {
@@ -1804,12 +1807,21 @@ select_entry_guard_for_circuit(guard_selection_t *gs,
     if (guard->is_reachable != GUARD_REACHABLE_NO) {
       *state_out = GUARD_CIRC_STATE_USABLE_ON_COMPLETION;
       guard->last_tried_to_connect = approx_time();
-      log_info(LD_GUARD, "Selected primary guard %s for circuit.",
-               entry_guard_describe(guard));
-      return guard;
+      smartlist_add(usable_primary_guards, guard);
+      if (smartlist_len(usable_primary_guards) >= num_entry_guards)
+        break;
     }
   } SMARTLIST_FOREACH_END(guard);
 
+  if (smartlist_len(usable_primary_guards)) {
+    entry_guard_t *guard = smartlist_choose(usable_primary_guards);
+    smartlist_free(usable_primary_guards);
+    log_info(LD_GUARD, "Selected primary guard %s for circuit.",
+             entry_guard_describe(guard));
+    return guard;
+  }
+  smartlist_free(usable_primary_guards);
+
   /* "Otherwise, if the ordered intersection of {CONFIRMED_GUARDS}
       and {USABLE_FILTERED_GUARDS} is nonempty, return the first
       entry in that intersection that has {is_pending} set to