Fix issue with request flags leak between tests

Reset request flags of RTNLHandler on Stop().  Make Stop() private, but
call it in teardown of all unit tests that call Start().

BUG=chromium-os:18833
TEST=Rerun unit tests

Change-Id: I83d32abc561e80c9dc72239825907cb5d5155bca
Reviewed-on: http://gerrit.chromium.org/gerrit/5461
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/rtnl_handler.cc b/rtnl_handler.cc
index cd4d9f8..8fd4167 100644
--- a/rtnl_handler.cc
+++ b/rtnl_handler.cc
@@ -92,6 +92,7 @@
   sockets_->Close(rtnl_socket_);
   in_request_ = false;
   sockets_ = NULL;
+  request_flags_ = 0;
   VLOG(2) << "RTNLHandler stopped";
 }