Update for mbox format.

Update for mbox format.
Various small fixes, as well as the following.
(Out of Rmail): Node deleted.
(Rmail): Update menu.
(Rmail Files): Comment out set-rmail-inbox-list.
Document rmail-inbox-list instead.
(Rmail Output): Substantial changes since C-o is now
rmail-output-as-seen and o is rmail-output.
(Rmail Attributes): Delete `stored', add `retried'.
(Rmail Display): Editing headers works in all cases.