Edebug: Disable backtracking when hitting a &define keyword.

Edebug: Disable backtracking when hitting a &define keyword.

Edebug doesn't deal well with backtracking out of definitions, see
Bug#41988.  Rather than trying to support this rare situation (e.g. by
implementing a multipass parser), prevent it by adding an implicit
gate.

* lisp/emacs-lisp/edebug.el (edebug--match-&-spec-op): Disable
backtracking when hitting a &define keyword.

* test/lisp/emacs-lisp/edebug-tests.el
(edebug-tests-duplicate-&define): New unit test.
(edebug-tests--duplicate-&define): New helper macro.

* doc/lispref/edebug.texi (Backtracking): Mention &define in the list
of constructs that disable backtracking.

* etc/NEWS: Document new behavior.