Add filter to register_post_type $args list

[maxaud]

The ability to filter register_post_type args list would be nice. Some plugins and themes add custom post types with horrible named slugs (ex: mydomain.com/mytheme_portfolio/<post_title>).

I currently don't see a way to modify the rewrite argument to add a customized slug.

[scribu]

I suggested something similar in #21944.

Not sure if it would make sense to add it for custom post types only.

[scribu]

#17447