LWIP_NETIF_HOSTNAME macro

LWIP_NETIF_HOSTNAME==1: use DHCP_OPTION_HOSTNAME with netif's hostname field. LWIP_DHCP_DISCOVER_ADD_HOSTNAME==1: include hostname opt in discover packets. If the hostname is not set in the DISCOVER packet, then some servers might issue an OFFER with hostname configured and consequently reject the REQUEST with any other hostname.