New macro macroexp-let2*

* emacs-lisp/macroexp.el (macroexp-let2*): New macro.

* window.el (with-temp-buffer-window)
(with-current-buffer-window, with-displayed-buffer-window):
* emacs-lisp/cl-macs.el (cl--compiler-macro-adjoin):
* emacs-lisp/cl-lib.el (substring):
* emacs-lisp/cl-extra.el (cl-getf): Use it.
* net/eww.el (eww-set-character-encoding): Use `read-coding-system'.
* processes.texi (Network Security): Made into its own section and fleshed out