Change Edebug's behavior-changing hooks to variables

Change Edebug's behavior-changing hooks to variables

* lisp/emacs-lisp/edebug.el (edebug-after-instrumentation-functions)
(edebug-new-definition-functions): Deleted.
(edebug-after-instrumentation-function)
(edebug-new-definition-function): New variables.
(edebug-behavior-alist): Update docstring.
(edebug-read-and-maybe-wrap-form1, edebug-make-form-wrapper): Use new
variables.

* lisp/emacs-lisp/testcover.el (testcover-start)
(testcover-this-defun): Use `edebug-after-instrumentation-function' and
`edebug-new-definition-function'.
(testcover-after-instrumentation): Return passed form.
(testcover-init-definition): Use argument instead of `edebug-def-name'.

* doc/lispref/edebug.texi (Edebug Options): Replace descriptions of
`edebug-after-instrumentation-functions' and `edebug-new-definition-functions'
with `edebug-after-instrumentation-function' and
`edebug-new-definition-function'.