Never delete Subnets when StrictSubnets is set

If a node is unreachable, and not connected to an edge anymore, it gets
deleted. When this happens its subnets are also removed, which should
not happen with StrictSubnets=yes.

Solution:
- do not remove subnets in src/net.c::purge(), we know that all subnets
  in the list came from our hosts files.
  I think here you got the check wrong by looking at the tunnelserver
  code below it - with strictsubnets we still inform others but do not
  remove the subnet from our data.
- do not remove nodes in net.c::purge() that still have subnets
  attached.

src/net.c

@@ -68,8 +68,8 @@ static void purge(void) {
 			for(snode = n->subnet_tree->head; snode; snode = snext) {
 				snext = snode->next;
 				s = snode->data;
-				if(!strictsubnets)
 				send_del_subnet(broadcast, s);
+				if(!strictsubnets)
 					subnet_del(n, s);
 			}
 
@@ -98,7 +98,8 @@ static void purge(void) {
 					break;
 			}
 
-			if(!enode)
+			if(!enode && (!strictsubnets || !n->subnet_tree->head))
+				/* in strictsubnets mode do not delete nodes with subnets */
 				node_del(n);
 		}
 	}