Update header.

Update header.
Don't require 'poe; it is required implicitly via 'mime-def.
(pack-sequence): Moved to top of the function definitions.
(base64-encode-string): Calculate padding first.
(base64-internal-encode-region): Changed order of evaluation to
"encode, delete, then insert".
(base64-internal-decode-string): Changed order of evaluation to
"decode, delete, then insert".
(base64-encode-region): Use read-only `interactive' spec.
(base64-decode-region): Ditto.
(base64-decode-string): Not interactive.
(encoded-text-decode-string): Anchor regexp with "\\`" and "\\'".
(base64-insert-encoded-file): Use built-in `interactive' spec.
(base64-write-decoded-region): Ditto.