Add has_password and post_password query variables to WP_Query.

 * has_password true means posts with passwords, false means posts without.
 * post_password can query for posts with a particular password.

props wonderboymusic, robmiller.
fixes #20308.


git-svn-id: https://develop.svn.wordpress.org/trunk@27395 602fd350-edb4-49c9-b593-d223f7449a82