[dhcp] Add PXE-mandated DHCP options [128,135] to parameter request list

The PXE specification requires us to request DHCP options 128 to 135
inclusive, although these have no defined purpose.

Suggested-by: Ralf Buettner <rab@bootix.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/dhcp.c

 		      DHCP_LOG_SERVERS, DHCP_HOST_NAME, DHCP_DOMAIN_NAME,
 		      DHCP_ROOT_PATH, DHCP_VENDOR_ENCAP, DHCP_VENDOR_CLASS_ID,
 		      DHCP_TFTP_SERVER_NAME, DHCP_BOOTFILE_NAME,
+		      128, 129, 130, 131, 132, 133, 134, 135, /* for PXE */
 		      DHCP_EB_ENCAP, DHCP_ISCSI_INITIATOR_IQN ),
 	DHCP_END
 };