fix icmp type/code 3/0 response handling to port scans (port is filtered)

CHANGELOG

@@ -1,5 +1,9 @@
 # Nmap Changelog ($Id$)
 
+o Better handle ICMP type 3, code 0 (network unreachable) responses to
+  port scan packets.  These are rarely seen when scanning hosts that
+are actually online, but are still worth handling.
+
 o Fixed a problem with the -S and option on Windows reporting "Failed
   to resolve/decode supposed IPv4 source address".  The -D (decoy)
   option was probably broken on that platform too.  Thanks to kx

scan_engine.cc

@@ -2798,6 +2798,9 @@ static bool get_pcap_result(UltraScanInfo *USI, struct timeval *stime) {
 	    break;
 	  }
 	  switch(icmp->icmp_code) {
+	  case 0: /* Network unreachable */
+	    newstate = PORT_FILTERED;
+	    break;
 	  case 1: /* Host Unreachable */
 	    newstate = PORT_FILTERED;
 	    break;