* lisp/mh-e: Enable lexical-binding in all the files

* lisp/mh-e: Enable lexical-binding in all the files

* lisp/mh-e/mh-alias.el: Use lexical-binding.
(mh-alias-insert-file): Completion tables can be mere lists of strings.

* lisp/mh-e/mh-mime.el: Use lexical-binding.
(mh-mm-inline-message): Remove always-nil var `visible-headers`.

* lisp/mh-e/mh-search.el: Use lexical-binding.
(mh-search-folder): Remove unused var `pick-folder`.

* lisp/mh-e/mh-show.el: Use lexical-binding.
(mh-display-msg): Remove always-nil var `visible-headers`.

* lisp/mh-e/mh-utils.el: Use lexical-binding.
(completion-root-regexp): Always declare var.

* lisp/mh-e/mh-buffers.el: Use lexical-binding.
* lisp/mh-e/mh-comp.el: Use lexical-binding.
* lisp/mh-e/mh-folder.el: Use lexical-binding.
* lisp/mh-e/mh-funcs.el: Use lexical-binding.
* lisp/mh-e/mh-gnus.el: Use lexical-binding.
* lisp/mh-e/mh-identity.el: Use lexical-binding.
* lisp/mh-e/mh-inc.el: Use lexical-binding.
* lisp/mh-e/mh-junk.el: Use lexical-binding.
* lisp/mh-e/mh-letter.el: Use lexical-binding.
* lisp/mh-e/mh-limit.el: Use lexical-binding.
* lisp/mh-e/mh-print.el: Use lexical-binding.
* lisp/mh-e/mh-scan.el: Use lexical-binding.
* lisp/mh-e/mh-seq.el: Use lexical-binding.
* lisp/mh-e/mh-speed.el: Use lexical-binding.
* lisp/mh-e/mh-thread.el: Use lexical-binding.
* lisp/mh-e/mh-tool-bar.el: Use lexical-binding.
* lisp/mh-e/mh-xface.el: Use lexical-binding.