Creating a new site introduces post ID collisions

[carlomanf]

When a new site is added to a network, the shared auto-increment needs to be applied before the "hello world" and "sample page" entries are created, or there is a good chance of introducing new post ID collisions.

[carlomanf]

Further to this, it appears that on networks that already have a post with ID number 1, the "wapuu" comment gets incorrectly posted to that post, rather than to the intended new post.

[carlomanf]

Removing this from the v1.2.0 milestone because there are hard-coded values and poor availability of filters in the wp_install_defaults function. It looks like the only reliable way to fix this at the moment is to "plug" the whole function.