Update header.

Update header.
Require 'path-util and don't require 'poem.
(mel-use-module): Eliminate local variable `encoding'.
(mime-insert-encoded-file for mel-b-builtin):
Use built-in `interactive' spec.
(encoded-text-decode-string): Anchor regexp with "\\`" and "\\'".
(mime-encode-region): Capitalize prompt string.
(mime-decode-region): Ditto.
(mime-insert-encoded-file): Ditto.
(mime-write-decoded-region): Ditto.