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also if `rmail-displayed-headers' is set.
(rmail-clear-headers): Handle `rmail-displayed-headers' correctly.

(rmail-insert-inbox-text): When reading old
.newmail-<user> file, report it specifically.

(rmail-displayed-headers): New variable.
(rmail-clear-headers): Handle rmail-displayed-headers

(rmail-display-labels): Use `format' to convert
message numbers to strings.

(rmail-resend): If mailabbrev loaded, use that to expand abbrevs.