Update header.

Update header.
Don't require 'poe; it is required implicitly via 'mime-def.
(base64-dl-handle): Chech whether `base64-dl-module' is string.
Defalias `base64-encode-string' and `base64-decode-string' just
after `dynamic-call'ing "emacs_base64_init".
(base64-encode-region): Use read-only `interactive' spec.
Use `base64-encode-string'.
Changed order of evaluation to "encode, delete, then insert".
(base64-decode-region): Renamed from `decode-base64-region'.
Use read-only `interactive' spec.
Use `base64-decode-string'.
Changed order of evaluation to "decode, delete, then insert".
(mime-encode-string): Use `base64-encode-string'.
(mime-decode-string): Use `base64-decode-string'.
(mime-decode-region): Use `base64-decode-region'.
(encoded-text-encode-string): Use `base64-encode-string'.
(encoded-text-decode-string): Anchor regexp with "\\`" and "\\'".
(mime-insert-encoded-file): Use built-in `interactive' spec.
(mime-write-decoded-region in comment): Ditto.