Description of Problem:
I am a long-time exmh user, and today was my annual attempt to wean myself
off it onto something different. I have 1.2GiB of mail in MH mailboxen.
Only once I'm happy with a new mailer can I sensibly consider converting it
to a different storage format, of course.

After configuring Evolution to find my ~/Mail directory, it got very slow
and unresponsive. All folders were displayed in the index, but only one or
two actually appeared to display any mail when I clicked on them -- others
just showed empty indices. The system became very sluggish at this point --
tcpdump showed there was a _lot_ of traffic to the NFS server containing my
home directory.

More investigation of the tcpdump showed that Evolution was very busy going
through the whole of my mail archive and corrupting it -- ostensibly adding
'X-Evolution:' headers to _every_ mail within it, but also stripping
whitespace and newlines from all the message headers while it was at it.

MH mailboxes don't have a way of storing message status other than the
'unseen sequence' kept in .mh_sequences in each folder -- it's perhaps
sensible for Evolution to add its own X-Evolution header to messages for
which it _changes_ the status. But to go through an entire mail archive
adding X-Evolution headers indicating the _default_ state, and to _ignore_
the unseen sequence, as it did, is somewhat broken. Corrupting the message
headers is even worse.


Steps to reproduce the problem:
1. Collect 1.2GiB of mail archive in MH mailboxen.
2. Point evo at it.

Actual Results:
Restore from backup.

Expected Results:
1. Evo should only add X-Evolution headers when a mail's status is
_changed_, not before.
2. Evo should not change the mail in any way other than to add a single
X-Evolution header.
3. Evo should honour (and update) the unseen sequence in the .mh_sequences
file, for MH mailboxen.
4. Whatever causes the apparently-empty indices should display a 'please
wait while I corrupt your mail archive' message, or other message as
appropriate for whatever task it's _actually_ waiting to complete.