When a network device is specified to tcpip_tx() or it's children, treat

it as a fallback network device rather than an explicitly forced network
device.

Clear routing table entries before re-attempting DHCP.

src/net/udp.c

@@ -143,7 +143,7 @@ int udp_senddata ( struct udp_connection *conn ) {
  *
  * @v conn		UDP connection
  * @v peer		Destination address
- * @v netdev		Net device via which to send (or NULL)
+ * @v netdev		Network device to use if no route found, or NULL
  * @v data		Data to send
  * @v len		Length of data
  * @ret rc		Return status code