Merge is_installed() into wp_not_installed() and is_blog_installed()

[nacin]

wp_not_installed() checks is_blog_installed(), and if we're not installed (or installing), we die on error.

The MU/MS function is_installed() runs when not installing and when !$current_site && !current_blog.

We should be able to merge the largely overlapping functionality quite a bit and remove is_installed() entirely.

Also, we might want to consider introducing multisite tables to the list of tables to check for in is_blog_installed(), and/or (idea for another ticket:) allow repair.php to help when multisite.

We also don't need to check tables in $wpdb->old_tables, so if nothing else comes out of this ticket then that should go in. repair.php has a simple if ( in_array( $table, $wpdb->old_tables ) ) continue;, and the functionality should be consistent.

We should also check for $wpdb->site instead of or in addition to $wpdb->options if multisite, the way is_installed() does.