avoid_blog_page_permalink_collision() should also check for attachments and custom post types

[Denis-de-Bernardy]

avoid_blog_page_permalink_collision() only checks for collisions with static pages.

to avoid further potential collisions, it should add an additional filter, to start with. and it should also try:

• attachments in the event the main blog uses the /%postname%/ structure.
• categories, tags, etc in the event a plugin strips their respective stubs
• anything that is not a post, page, attachment or revision, and which has no parent.

[wpmuguru]

If custom permalinks are using in MU folder install /blog is added to the beginning of the permalinks base (ex. /blog/%postname%/) on the main blog.

The only collision that would occur is with pages when default permalinks are used.

[wpmuguru]

This issue would become relevant if the /blog/ restriction in #12002 is addressed. Closing this as a duplicate.