Handle editing of header fields.

Handle editing of header fields.
(rmail-old-headers): New variable.
(rmail-edit-current-message): Set it, recording current headers.
(rmail-cease-edit): Compute new headers and diff against old ones.
Update the mbox buffer with the changes that were made.
(rmail-edit-headers-alist): New function.
(rmail-edit-diff-headers, rmail-edit-update-headers): New functions.