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.

discovery/gossiper_test.go

@@ -582,7 +582,7 @@ func createUpdateAnnouncement(blockHeight uint32,
 			BlockHeight: blockHeight,
 		},
 		Timestamp:       timestamp,
-		MessageFlags:    lnwire.ChanUpdateOptionMaxHtlc,
+		MessageFlags:    lnwire.ChanUpdateRequiredMaxHtlc,
 		ChannelFlags:    flags,
 		TimeLockDelta:   uint16(prand.Int63()),
 		HtlcMinimumMsat: htlcMinMsat,