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/dummy_burst_filter_impl.cc b/lib/flow_control/dummy_burst_filter_impl.cc
index 33c24cb..4241111 100644
--- a/lib/flow_control/dummy_burst_filter_impl.cc
+++ b/lib/flow_control/dummy_burst_filter_impl.cc
@@ -60,7 +60,8 @@
     dummy_burst_filter_impl::dummy_burst_filter_impl()
       : gr::block("dummy_burst_filter",
               gr::io_signature::make(0, 0, 0),
-              gr::io_signature::make(0, 0, 0))
+              gr::io_signature::make(0, 0, 0)),
+        d_filter_policy(FILTER_POLICY_DEFAULT)
     {
         message_port_register_in(pmt::mp("in"));        
         message_port_register_out(pmt::mp("out"));
@@ -75,6 +76,14 @@
 
     void dummy_burst_filter_impl::process_burst(pmt::pmt_t msg)
     {
+        if (d_filter_policy == FILTER_POLICY_DROP_ALL)
+          return;
+
+        if (d_filter_policy == FILTER_POLICY_PASS_ALL) {
+          message_port_pub(pmt::mp("out"), msg);
+          return;
+        }
+
         pmt::pmt_t header_plus_burst = pmt::cdr(msg);
         int8_t * burst = (int8_t *)(pmt::blob_data(header_plus_burst)) + sizeof(gsmtap_hdr);
         size_t burst_len = pmt::blob_length(header_plus_burst) - sizeof(gsmtap_hdr);
@@ -100,6 +109,20 @@
         }
         return true;
     }
+
+    /* Filtering policy */
+    filter_policy
+    dummy_burst_filter_impl::get_policy(void)
+    {
+      return d_filter_policy;
+    }
+
+    filter_policy
+    dummy_burst_filter_impl::set_policy(filter_policy policy)
+    {
+      d_filter_policy = policy;
+      return d_filter_policy;
+    }
     
   } /* namespace gsm */
 } /* namespace gr */