wireproto: move wireproto capabilities computation in a subfunction It will help people that need to add capabilities (in a more subtle was that just adding some to the list) in multiple way: 1. This function returns a list, not a string. Making it easier to look at, extend or alter the content. 2. The original capabilities function will be store in the dictionary of wire protocol command. So extension that wrap this function also need to update the dictionary entry. Both wrapping and update of the dictionary entry are needed because the `hello` wire protocol use the function itself. This is specifically sneaky for extension writer as ssh use the `hello` command while http use the `capabilities` command. With this new `_capabilities` function there is one and only one obvious place to wrap when needed.