This adds one line with the onion and the channel_update we extract from
it. This in turn allows us to check that the channel_update in the onion is not
type prefixed, and that we patch it correctly before passing it to gossipd.
The easiest way to do this is to play with the 'wallet_tx' semantics
and have 'amount' have meaning even when 'all_funds' is set.
Note that we change the string 'Cannot afford funding transaction' to
'Cannot afford transaction' as this code is also used for withdrawls.
Inspired-by: molz on #c-lightning
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
