Use correct source port for NBNS spoofer

137 is only correct for systems that use this as their source port.
Systems running Samba, for example, don't use this.  So use the port
taken from the original request, not 137 or 1337

modules/auxiliary/spoof/nbns/nbns_response.rb

@@ -67,6 +67,7 @@ class Metasploit3 < Msf::Auxiliary
 
     while @run # Not exactly thrilled we can never turn this off XXX fix this sometime.
       packet, addr = @sock.recvfrom(512)
+      src_port = addr[1]
       rhost = addr[3]
 
       break if packet.length == 0
@@ -127,7 +128,7 @@ class Metasploit3 < Msf::Auxiliary
         p.ip_daddr = rhost
         p.ip_ttl = 255
         p.udp_sport = 137
-        p.udp_dport = 137
+        p.udp_dport = src_port
         p.payload = response
         p.recalc