Set post password cookies via an action in wp-login.php. Retire wp-pass.php...

Set post password cookies via an action in wp-login.php. Retire wp-pass.php (one less root file). Obey login ssl preferences for post password form submission. Props SergeyBiryukov. fixes #19798

git-svn-id: https://core.svn.wordpress.org/trunk@19925 1a063a9b-81f0-0310-95a4-ce76da25c4cd