Fix pivot handler to not consume all packets

Packet handlers should only return true if they consume a packet.
Otherwise, they should return false so something else can consume it.
This fixes port forwards by allowing the socket handler to see packets
that were otherwise being discarded in the pivot handler.

lib/rex/post/meterpreter/pivot.rb

@@ -45,11 +45,14 @@ class Pivot
     # Class request handler for all channels that dispatches requests
     # to the appropriate class instance's DIO handler
     def request_handler(client, packet)
+      handled = false
       if packet.method == 'core_pivot_session_new'
+        handled = true
         session_guid = packet.get_tlv_value(TLV_TYPE_SESSION_GUID)
         listener_id = packet.get_tlv_value(TLV_TYPE_PIVOT_ID)
         client.add_pivot_session(Pivot.new(client, session_guid, listener_id))
       elsif packet.method == 'core_pivot_session_died'
+        handled = true
         session_guid = packet.get_tlv_value(TLV_TYPE_SESSION_GUID)
         pivot = client.find_pivot_session(session_guid)
         if pivot
@@ -57,7 +60,7 @@ class Pivot
           client.remove_pivot_session(session_guid)
         end
       end
-      true
+      handled
     end
   end