Form validation for password resets.

Merges [30417] to the 4.0 branch.

Built from https://develop.svn.wordpress.org/branches/4.0@30418


git-svn-id: http://core.svn.wordpress.org/branches/4.0@30413 1a063a9b-81f0-0310-95a4-ce76da25c4cd