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.

lntest/itest/lnd_multi-hop-payments.go

@@ -48,7 +48,7 @@ func testMultiHopPayments(net *lntest.NetworkHarness, t *harnessTest) {
 	//
 	// First, we'll create Dave and establish a channel to Alice. Dave will
 	// be running an older node that requires the legacy onion payload.
-	daveArgs := []string{"--protocol.legacyonion"}
+	daveArgs := []string{"--protocol.legacy.onion"}
 	dave, err := net.NewNode("Dave", daveArgs)
 	if err != nil {
 		t.Fatalf("unable to create new nodes: %v", err)

lntest/itest/lnd_test.go

@@ -1178,7 +1178,7 @@ func (c commitType) String() string {
 func (c commitType) Args() []string {
 	switch c {
 	case commitTypeLegacy:
-		return []string{"--protocol.committweak"}
+		return []string{"--protocol.legacy.committweak"}
 	case commitTypeTweakless:
 		return []string{}
 	case commitTypeAnchors: