Update the global o.numhosts_scanning variable within ultra_scan, so

that it is set properly for ping scan. Previously status reports during
ping scan always looked like

Stats: 25:34:33 elapsed; 991232 hosts completed (72530 up), 0 undergoing Ping Scan

Note the "0 undergoing". Now that number will be set to 4096 or whatever
the current ping scan group size happens to be.

scan_engine.cc

@@ -4873,6 +4873,9 @@ void ultra_scan(vector<Target *> &Targets, struct scan_lists *ports,
   }
 #endif
 
+  // Set the variable for status printing
+  o.numhosts_scanning = Targets.size();
+
   startTimeOutClocks(Targets);
   USI = new UltraScanInfo(Targets, ports, scantype);