Quieten cl-lib related compiler warnings

Quieten cl-lib related compiler warnings

* lisp/completion.el (cl-set-difference):
* lisp/files.el (map-merge-with, map-merge):
* lisp/emacs-lisp/radix-tree.el (map-apply): Declare.
* lisp/emacs-lisp/thunk.el: Load cl-lib at run-time, not
cl-macs at compile.
* lisp/gnus/gnus-group.el: Load cl-lib at run-time, not cl at compile.
* lisp/emacs-lisp/checkdoc.el, lisp/emacs-lisp/package.el
* lisp/gnus/gnus-sum.el, lisp/gnus/message.el, lisp/net/shr.el:
Load cl-lib at run-time.
* lisp/gnus/mml-sec.el (mml-signencrypt-style)
(mml-secure-cust-record-keys): Replace cl with cl-lib,
and load it at run-time.
* lisp/cedet/ede/linux.el, lisp/vc/vc-hg.el: Reorder requires.