in order to preserve current behavior, we cap at max if specified 'all';
otherwise we fail since the amount requested is larger than the
channel max capacity
Previously, we'd fail on 'all'. `fundchannel_start` needs an amount
in order to start a funding transaction.
The way that we approach this is to first call `txprepare` with a
placeholder address and the 'all' amount; this will return the maximum
amount available. We then clamp this to the max_funding (currently
hardcoded, in the future we'd want to consult our/the peer's features) and
then use the amount on the output in the prepared transaction as the
funding amount. We then pass this amount to fundchannel_start,
after we've started it successfully we cancel the held placeholder
transaction and prepare a second transaction for the exact amount,
using the funding address that fundchannel_start passed back.
lisa neigut