Fix semantics of let-binding `process-environment'.

Fix semantics of let-binding `process-environment'.

* lisp/env.el: Require cl for byte compilation. (For `block' and `return'.)
  (read-envvar-name): Update for rename.  Include `process-environment'
  as well.
  (setenv): Update for rename also handle `process-environment'.  Update doc.
  (getenv): Update doc.
  (environment): New function.
  (let-environment): New macro.

* lisp/font-lock.el (lisp-font-lock-keywords-2): Add `let-environment'.

* src/callproc.c (Vglobal_environment): New variable, taking over the
  previous role of `Vprocess_environment', which is now something else.
  (add_env): New function.
  (child_setup): Use it.
  (child_setup, getenv_internal): Rename Vprocess_environment to
  Vglobal_environment.  Handle the new Vprocess_environment.
  (Fgetenv_internal, egetenv): Update doc.
  (set_process_environment): Rename to `set_global_environment'.  Rename
  Vprocess_environment to Vglobal_environment.
  (syms_of_callproc): Rename process-environment to global-environment,
  add new process-environment, update docs.

* src/emacs.c (main): Call set_global_environment instead of
  set_process_environment.

* fileio.c (Fread_file_name): Update comment.

git-archimport-id: lorentey@elte.hu--2004/emacs--multi-tty--0--patch-465