(base64-internal-decoding-limit): Switch default value between

(base64-internal-decoding-limit): Switch default value between
XEmacs-mule and other emacsen.
Abolish function `base64-decode-string!'.
(base64-internal-decode-region): New implementation.
(base64-insert-encoded-file): New function.
(mime-insert-encoded-file): Use `base64-insert-encoded-file'.
(base64-write-decoded-region): New function.
(mime-write-decoded-region): Use `base64-write-decoded-region'.