Create envelope->changed to mark all field changes.

Create envelope->changed to mark all field changes.

In subsequent commits, we're going to add the x-label and subject
headers changed flags into the envelope.  To avoid the list of checks
exploding everywhere, just use a single field to check and reset those
values.

Several places in the code are checking for a null header->env.  I
wasn't aware this was possible, so I've added todo notes to track down
when this occurs.