Handle under-coin-selecting due to an added OP_RETURN output

Handle under-coin-selecting due to an added OP_RETURN output

When we do coin selection for channel close anchor spends, we may
do coin selection targeting exactly the input values we need.
However, if coin selection does not include a change output, we may
add an OP_RETURN output, which may cause us to end up with less
fee than we wanted on the resulting package.

Here we address this issue by running coin selection twice - first
without seeking the extra weight of the OP_RETURN output, and again
if we find that we under-selected.