Check for all dust is too strict

[JssDWt]

Issue and Steps to Reproduce

• Spin up two core lightning nodes, Alice and Bob
• Alice allows dust reserve, Bob does not
• Alice funds a zero reserve zero conf channel with Bob
• Bob will now have a reserve of 0, but Alice will have a reserve of 1% of the channel capacity.
• There is no way the commitment transaction will ever have zero outputs, because Alice is the funder and her side of the channel will have a reserve above the dust limit.

Currently, this gives the following error:
channel funding 100000sat too small for chosen parameters: a total of 573 HTLCs with dust value 546sat would result in a commitment_transaction without outputs. Please increase the funding amount or reduce the max_accepted_htlcs to ensure at least one non-dust output.

I'd expect this flow to give no error, as long as Alice's reserve is above the dust limit.

Relevant code: https://github.com/ElementsProject/lightning/blob/master/openingd/openingd.c#L481-L520