Make seek information part of the xfer metadata, rather than an entirely

separate xfer method.

Add missing .alloc_iob entries to several xfer_interface_operations
structures.

src/net/udp/dhcp.c

@@ -927,8 +927,8 @@ static int dhcp_deliver_raw ( struct xfer_interface *xfer,
 static struct xfer_interface_operations dhcp_xfer_operations = {
 	.close		= ignore_xfer_close,
 	.vredirect	= xfer_vopen,
-	.seek		= ignore_xfer_seek,
 	.window		= unlimited_xfer_window,
+	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= dhcp_deliver_raw,
 };