(mime-encoding-list): New user option (renamed from

(mime-encoding-list): New user option (renamed from
`mime-content-transfer-encoding-list').
(mime-encoding-list): New function.
(mime-encoding-alist): New function.
(mime-encode-region): Use `mime-encoding-alist' for completion.
(mime-decode-region): Use `mime-encoding-alist' for completion.
(mime-insert-encoded-file): Use `mime-encoding-alist' for completion.
(mime-write-decoded-region): Use `mime-encoding-alist' for
completion.