(pmailhdr): Don't require it.

(pmailhdr): Don't require it.
(pmail-fetch-field): Function deleted.
Callers use pmail-get-header.
(pmail-sort-messages): Replace point-offset with
return-to-point, which can be nil.
Call pmail-swap-buffers-maybe after that.
Don't bind buffer-read-only.  Bind inhibit-read-only.
Be more careful in making blank line at end of msg.
Don't enable undo at end.