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/include/grgsm/flow_control/burst_sdcch_subslot_filter.h b/include/grgsm/flow_control/burst_sdcch_subslot_filter.h
index 5bcf829..a68782f 100644
--- a/include/grgsm/flow_control/burst_sdcch_subslot_filter.h
+++ b/include/grgsm/flow_control/burst_sdcch_subslot_filter.h
@@ -25,6 +25,7 @@
 
 #include <grgsm/api.h>
 #include <gnuradio/block.h>
+#include <grgsm/flow_control/common.h>
 
 namespace gr {
   namespace gsm {
@@ -61,6 +62,10 @@
 
       virtual subslot_filter_mode get_mode(void) = 0;
       virtual subslot_filter_mode set_mode(subslot_filter_mode mode) = 0;
+
+      /* Filtering policy */
+      virtual filter_policy get_policy(void) = 0;
+      virtual filter_policy set_policy(filter_policy policy) = 0;
     };
   } // namespace gsm
 } // namespace gr