Fix marking "delayed-initialization" vars as dynamically scoped

Fix marking "delayed-initialization" vars as dynamically scoped

We used to mark those vars as dynbound in `custom-reevaluate-setting`
which forced us to bind `current-load-list` around it to avoid having
the vars be associated with the wrong file.  Move this marking to
`custom-initialize-delay` so we don't need this workaround.

* lisp/custom.el (custom-initialize-delay): Mark the var as dynamic.
(custom-reevaluate-setting): Don't use `defvar` here.

* lisp/startup.el (command-line): Don't let-bind `current-load-list`
around calls to `custom-reevaluate-setting`.