* lisp/mh-e: Use cl-lib

* lisp/mh-e: Use cl-lib

Also, use underscore prefixes and defvar in preparation for lexical binding

* lisp/mh-e/mh-acros.el: Require cl-lib instead of cl.
Rename all cl.el uses by adding `cl-` prefix.
(mh-require-cl): Remove.  Not needed any more.  Remove all calls.
(mh-defstruct): Remove.  Replace all uses with cl-defstruct.
(mh-dlet*): New macro.

* lisp/mh-e/mh-comp.el (mh-user-agent-compose): Fold all ignored
optional args into the &rest arg.

* lisp/mh-e/mh-e.el: Require cl-lib instead of using mh-require-cl.
(mh-variants): Don't add-to-list on a local var.

* lisp/mh-e/mh-folder.el (mh-restore-desktop-buffer): Use shorter arg
names that don't collide with global vars.

* lisp/mh-e/mh-mime.el (mh-insert-mime-button):
(mh-insert-mime-security-button): Use mh-dlet*.

* lisp/mh-e/mh-search.el (mh-swish-next-result, mh-grep-next-result)
(mh-namazu-next-result): Use `or`.

* lisp/mh-e/mh-thread.el (mh-thread-generate)
(mh-thread-prune-containers): Use underscore rather than declare+ignore.

* lisp/mh-e/mh-tool-bar.el (mh-tool-bar-define): Use mh-dlet*.
(mh-tool-bar-define): Prefer the more precise \`...\' regexp ops.
Prefer Elisp's `eval-and-compile` over `cl-eval-when`.

* lisp/mh-e/mh-xface.el (mh-picon-get-image): Don't use
mh-funcall-if-exists for ietf-drums-parse-address.
Avoid the use of `cl-return` and hence use plain `defun`.
Replace some `cl-loop` with `dolist`.