flow_control: implement pass / drop filtering policies

This change introduces a set of three modes for flow control
filters, one of which is default behavor and two extra modes
else described below:

  - FILTER_POLICY_PASS_ALL
  - FILTER_POLICY_DROP_ALL

Both modes are opposite, and make a filter either unconditionally
pass or drop all the data one gets to the input. They would be
usable for some external usage.
diff --git a/lib/flow_control/burst_fnr_filter_impl.h b/lib/flow_control/burst_fnr_filter_impl.h
index b113679..9d13825 100644
--- a/lib/flow_control/burst_fnr_filter_impl.h
+++ b/lib/flow_control/burst_fnr_filter_impl.h
@@ -34,6 +34,7 @@
     class burst_fnr_filter_impl : public burst_fnr_filter
     {
      private:
+      filter_policy d_filter_policy;
       unsigned int d_framenr;
       filter_mode d_mode;
      public:
@@ -47,6 +48,10 @@
 
       filter_mode get_mode(void);
       filter_mode set_mode(filter_mode mode);
+
+      /* Filtering policy */
+      filter_policy get_policy(void);
+      filter_policy set_policy(filter_policy policy);
     };
 
   } // namespace gsm