[Nmh-workers] Renaming Nmh-Attachment to Attach

[Ken Hornstein]

In preparation for making mhbuild run all of the time, I'd like to make
the following changes:

- Change the header name used by the attachment code from "Nmh-Attachment"
  to "Attach".  I'm thinking people might want to add the header themselves,
  and I don't see the value in the longer name.  Also, some compatibility
  with mutt can't hurt.

- Make it non-selectable.  The current options to pick the header name would
  result in a warning being issued.  Next release after 1.6 they would be
  removed.

Objections?

--Ken