Move the heavy lifting done in install.php and upgrade.php into functions and make those functions pluggable. Custom WP distro makers and i18n teams have requested pluggable install functions. So, let's add these:
- wp_install() - top level install function
- wp_install_defaults() - Installs default post, page, links, cat, etc.
- wp_new_blog_notification() - Sends the new blog email.
- wp_upgrade() - Top level upgrade function.
We might also want to make the schema pluggable. Could be handy for those porting to other DBs or for those who want to have extra fields and tables out of the box.