Add the StrictSubnets option.

When this option is enabled, tinc will not accept dynamic updates of Subnets
from other nodes, but will only use Subnets read from local host config files
to build its routing table.

src/net.c

@@ -68,7 +68,7 @@ static void purge(void) {
 			for(snode = n->subnet_tree->head; snode; snode = snext) {
 				snext = snode->next;
 				s = snode->data;
-				if(!tunnelserver)
+				if(!strictsubnets)
 					send_del_subnet(broadcast, s);
 				subnet_del(n, s);
 			}