Add a function free_services that tells Nmap to reload the nmap-services

file again when it needs it. This is called from nmap_free_mem, because
cp_free (also called by nmap_free_mem) invalidates members of the
services data structures.

In normal use this doesn't matter. It only matters when reinvoking the
engine several times with --interactive.

nmap.cc

@@ -1993,6 +1993,7 @@ void nmap_free_mem() {
   cp_free();
   free_dns_servers();
   free_etchosts();
+  free_services();
   if (o.reference_FPs) {
     delete o.reference_FPs;
     o.reference_FPs = NULL;