lncfg: split off protocol options into normal and legacy, normal reqs no build tag

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.

lncfg/protocol_experimental_on.go

@@ -0,0 +1,17 @@
+// +build dev
+
+package lncfg
+
+// ExperimentalProtocol is a sub-config that houses any experimental protocol
+// features that also require a build-tag to activate.
+type ExperimentalProtocol struct {
+	// Anchors should be set if we want to support opening or accepting
+	// channels having the anchor commitment type.
+	Anchors bool `long:"anchors" description:"EXPERIMENTAL: enable experimental support for anchor commitments, won't work with watchtowers"`
+}
+
+// AnchorCommitments returns true if support for the anchor commitment type
+// should be signaled.
+func (l *ExperimentalProtocol) AnchorCommitments() bool {
+	return l.Anchors
+}