Don't automatically join a network if we already have a link.

The _ConfigureInterface() method is used as a backend for all
configuration tasks. That includes setting addresses manually or by
DHCP and changing flags, mtu or metric. Therefore we can't join
networks every time it is invoked. Instead we check for an existing
link first and only try to join if there is none yet.

src/servers/net/NetServer.cpp

@@ -533,7 +533,7 @@ NetServer::_ConfigureInterface(BMessage& message)
 	}
 
 	BNetworkDevice device(name);
-	if (device.IsWireless()) {
+	if (device.IsWireless() && !device.HasLink()) {
 		const char* networkName;
 		if (message.FindString("network", &networkName) == B_OK) {
 			// join configured network