Require option_anysegwit/Adapt API

[TheBlueMatt]

Now that option_anysegwit has been around for a while, we should adapt the API to fetch the shutdown script to (a) allow any witness script, (b) maybe merge it with get_destination_script, since the two do basically the same thing.

[TheBlueMatt]

Sadly we can't really require option_anysegwit any time soon since lnd still doesn't ship it. Still, we adapted the API so its much neater now.