Adds a "verify" API to the msg module.  This always returns success in
the default implementation, because checksums were not historically
enforced.  However it can be made more strict in a custom
implementation.

Expand the "Increment Sequence" boolean on transmit message functions to
be a more general "Update Header" boolean, so it can be used for other
fields like timestamps, checksums, or user-defined fields too.

Fix #2335, generalize update header parameter

Fix #2337, add msg verify capability