Replace `(function (lambda' by `(lambda' throughout.

Replace `(function (lambda' by `(lambda' throughout.
(rmail-sort-by-date, rmail-sort-by-subject)
(rmail-sort-by-author, rmail-sort-by-recipient)
(rmail-sort-by-correspondent, rmail-select-correspondent)
(rmail-sort-by-lines, rmail-sort-by-labels, rmail-sort-messages)
(rmail-make-date-sortable): Doc fixes.
(rmail-sort-by-correspondent): Downcase correspondents.
(rmail-sort-by-labels): Make it work.
(rmail-sort-messages): Restore undo if it was initially enabled.
Fix bobp/bolp typo that was adding a line on every sort.