Skip privileged pings against localhost on Windows.

Compare to this similar test in Nmap's scan_engine.cc:
  if (scantype != CONNECT_SCAN && Targets[0]->ifType() == devt_loopback) {
    log_write(LOG_STDOUT, "Skipping %s against %s because Windows does not support scanning your own machine (localhost) this way.\n", scantype2str(scantype), Targets[0]->NameIP());
    return;
  }

rndbit made a report--unfortunately we con't fix it, but we can make the
error message clearer.
http://seclists.org/nmap-dev/2013/q1/187

CHANGELOG

@@ -1,5 +1,9 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [Nping] Nping now skips localhost targets for privileged pings (with
+  an error message) because those generally don't work.
+  [David Fifield]
+
 o [Ncat] Ncat now keeps running in connect mode after receiving EOF
   from the remote socket, unless --recv-only is in effect.
   [Tomas Hozza]

nping/NpingTargets.cc

@@ -277,7 +277,13 @@ int NpingTargets::processSpecs(){
 			delete mytarget;
 			continue;
 		  }
-
+#ifdef WIN32
+		  if (rnfo.ii.device_type == devt_loopback){
+			outError(QT_2, "Skipping %s because Windows does not allow localhost scans (try --unprivileged).", mytarget->getTargetIPstr() );
+			delete mytarget;
+			continue;
+		  }
+#endif
 		   /* Determine next hop */
 		  if( rnfo.direct_connect ){
 			mytarget->setDirectlyConnected(true);