Improve is_blog_installed() check and add means of automatically repairing tables

[ryan]

Improve is_blog_installed() check so that it reflects when the database tables are in an insane state. Instead of getting confused and offering to scratch install WP again, it should put up message on admin page visits that links to a page that will attempt to automatically repair tables.

[westi]

Looks good.

I would rename the REPAIR define to something WP'y like WP_ALLOW_REPAIR for example.

[ryan]

Will do.

[azaozz]

Was thinking we could exit or maybe redirect to the dashboard when is_blog_installed is true and WP_REPAIRING is set (meaning someone is running repair.php directly). But that would prevent the user from repairing other tables.

Perhaps we need to cover two more user cases:

• allow admins to run repair.php for cases when wp_posts, wp_comments, wp_terms, etc. need repairs

• if wp_users or wp_usermeta fail, offer to try to repair them the same way as wp_options.

[ryan]

(In [11902]) is_blog_installed() improvements and repair.php. see #10728

[ryan]

First round committed. I'll work on azaozz's suggestions next.