Fix the expunge command so that the current message index is correct.

Fix the expunge command so that the current message index is correct.

* pmail.el (pmail-expunge-counter): New variable
  (pmail-set-attribute): Canonicalize the calling sequence to index, attribute, state.
  (pmail-show-message, pmail-delete-message, pmail-undelete-previous-message,
   pmail-delete-forward, pmail-mark-message): Use the new canonical calling sequence.
  (pmail-only-expunge): Use the expunge counter to update the current message index.
  (pmail-expunge-callback): Simplify.  Just count the expunged messages with a lower
   index than the current message index.

* pmaildesc.el (pmail-desc-get-previous): Fix an "off by one" issue.
  (pmail-desc-set-attribute): Canonicalize the calling sequence to index, attribute, state.

* pmailout.el (pmail-output-body-to-file): Use the canonical calling sequence.