In this commit, we split off the protocol options into a normal and
legacy sub-config. The legacy sub-config protected by a built tag, and
will only be populated if thet tag is set. Legacy options now have a
`legacy` prefix. So `--protocol.legacyonion` is now `--protocol.onion`,
and `--protocol.committweak`, is now `--protocol.legacy.committweak`.
We also create a new experimental protocol feature sub-config for newer
features that may not yet been fully complete, so they require a build
tag.
