Resurrect edebug-set-initial-mode, repurposing it to set the global mode.

Resurrect edebug-set-initial-mode, repurposing it to set the global mode.

lisp/emacs-lisp/edebug.el (edebug-initial-mode-alist): Uncomment, and
amend to match current modes and functions.
(edebug-set-initial-mode): Uncomment and change from setting a defun's
`edebug-initial-mode''s property to setting the variable
`edebug-initial-mode'.
(top level): Create new binding C-x C-a C-m for
`edebug-set-initial-mode'.

doc/lispref/edebug.texi (Edebug Execution Modes): document
`edebug-set-initial-mode' and its new key binding.
(Edebug Options): Mention the new command in the pertinent place.

etc/NEWS: Write entry for this change.