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.

lnrpc/devrpc/dev_server.go

@@ -311,7 +311,8 @@ func (s *Server) ImportGraph(ctx context.Context,
 				policy.MaxHTLC = lnwire.MilliSatoshi(
 					rpcPolicy.MaxHtlcMsat,
 				)
-				policy.MessageFlags |= lnwire.ChanUpdateOptionMaxHtlc
+				policy.MessageFlags |=
+					lnwire.ChanUpdateRequiredMaxHtlc
 			}
 
 			return policy