(custom--standard-value-p): New function

(custom--standard-value-p): New function

* lisp/cus-edit.el (custom-variable-state)
(custom-variable-mark-to-reset-standard): Use `custom--standard-value-p`.

* lisp/custom.el (custom--standard-value-p): New function.
(customize-mark-to-save, custom-push-theme, enable-theme): Use it.

* lisp/help-fns.el (describe-variable): Use `custom--standard-value`.
Extracted from `customize-mark-to-save` by with `ignore-errors`
replaced by `with-demoted-errors`.

(cherry picked from commit 71b3298c0e813ba1432e75370c460eea5caf72d5)