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.

funding/manager.go

@@ -3637,7 +3637,7 @@ func (f *Manager) newChanAnnouncement(localPubKey,
 
 	// Our channel update message flags will signal that we support the
 	// max_htlc field.
-	msgFlags := lnwire.ChanUpdateOptionMaxHtlc
+	msgFlags := lnwire.ChanUpdateRequiredMaxHtlc
 
 	// We announce the channel with the default values. Some of
 	// these values can later be changed by crafting a new ChannelUpdate.