multi: rename due to required maxHTLC bit

We rename `ChanUpdateOptionMaxHtlc` to `ChanUpdateRequiredMaxHtlc`
as with the latest changes it is now required.

Similarly, rename `validateOptionalFields` to
`ValidateChannelUpdateFields`, export it to use it in a later commit.

routing/unified_edges_test.go

@@ -25,7 +25,7 @@ func TestNodeEdgeUnifier(t *testing.T) {
 		FeeProportionalMillionths: 100000,
 		FeeBaseMSat:               30,
 		TimeLockDelta:             60,
-		MessageFlags:              lnwire.ChanUpdateOptionMaxHtlc,
+		MessageFlags:              lnwire.ChanUpdateRequiredMaxHtlc,
 		MaxHTLC:                   5000,
 		MinHTLC:                   100,
 	}
@@ -33,7 +33,7 @@ func TestNodeEdgeUnifier(t *testing.T) {
 		FeeProportionalMillionths: 190000,
 		FeeBaseMSat:               10,
 		TimeLockDelta:             40,
-		MessageFlags:              lnwire.ChanUpdateOptionMaxHtlc,
+		MessageFlags:              lnwire.ChanUpdateRequiredMaxHtlc,
 		MaxHTLC:                   4000,
 		MinHTLC:                   100,
 	}