(mail-abbrevs-mode): Use define-minor-mode.

(mail-abbrevs-mode): Use define-minor-mode.
(mail-abbrevs-setup): Use abbrev-expand-functions.
(build-mail-abbrevs): Use with-temp-buffer.
(define-mail-abbrev): Simplify.
(mail-abbrev-expand-wrapper): Rename sendmail-pre-abbrev-expand-hook.
Change it for use on abbrev-expand-functions.
(mail-abbrev-complete-alias): Use with-syntax-table.