Move check_password_reset_key(), reset_password(), and register_new_user() from wp-login.php to wp-includes/user.php, to make them reusable. props beaulebens for initial patch. fixes #20279.

git-svn-id: https://develop.svn.wordpress.org/trunk@25231 602fd350-edb4-49c9-b593-d223f7449a82