bug#49593: Emacs overrides normal-erase-is-backspace

[Paul W. Rankin]

Overview:

Emacs overrides user's customization of normal-erase-is-backspace with 
no clear way to set it.

Steps to reproduce:

1. insert into start.el:

    (custom-set-variables '(normal-erase-is-backspace t))

2. emacs -Q -l start.el

3. M-x customize-option RET normal-erase-is-backspace

Expected behaviour:

normal-erase-is-backspace is t.

Actual behaviour:

normal-erase-is-backspace is maybe; marked as "CHANGED outside of 
Customize"