fundingmanager: determine our dust limit based on active chain
fundingmanager: determine our dust limit based on active chain

In this commit, we fix an issue where we would always assume the dust
limit was Bitcoin's dust limit, rather than the active chain. This would
lead to issues when attempting to open channels on the Litecon chain.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd