(Re-)add handling for `ChannelUpdate::message_flags`

(Re-)add handling for `ChannelUpdate::message_flags`

When the `htlc_maximum_msat` field was made mandatory in
`ChannelUpdate` (in b0e8b739b73cc25f3e1ab00695a14d972162a140) we
started ignoring the `message_flags` field entirely and always
writing `1`. The comment updates indicated that the `message_flags`
field was deprecated, but this is not true - only the
`htlc_maximum_msat` indicator bit was deprecated, requiring it to
be 1.

If a node creates a `channel_update` with `message_flags` bits set
other than the low bit, this will cause us to spuriously reject
the message with an invalid signature error as we will check the
message against the wrong hash.

With today's current spec this is totally fine - the only other bit
defined for `message_flags` is the `dont_forward` bit, which when
set indicates we shouldn't accept the message into our gossip store
anyway (though this may lead to spurious `warning` messages being
sent to peers). However, in the future this may mean we start
rejecting valid `channel_update`s if new bits are defiend.