Use ";; Shush compiler." comment consistently per Mark's suggestion.

Use ";; Shush compiler." comment consistently per Mark's suggestion.
(mh-letter-mode): Derive from mail-mode and delete code copied from
mail-mode (closes SF #1385571). Mention mail-mode-hook in docstring.
(mh-fill-paragraph-function): Delete. Handled by mail-mode.
(mh-to-field-choices): For consistency with mail-mode, add "a" for
"Mail-Reply-To:", "l" for "Mail-Followup-To:", and "r" for
"Reply-To:". Change "r" to "m" for "From:" (closes SF #1400139).
(mh-to-fcc): Move setting of folder argument to interactive argument
as is the norm.
(mh-letter-complete-function-alist): Add mail-reply-to.
(mh-letter-mode-map): Add keys for mh-to-field (Mail-Reply-To,
Mail-Followup-To, Reply-To, From).