Use prepare() for the duplicate comment query in wp_allow_comment().

see #21767


git-svn-id: http://core.svn.wordpress.org/trunk@23973 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use API instead of bare SQL queries in site-users.php.

see #21767


git-svn-id: http://core.svn.wordpress.org/trunk@23974 1a063a9b-81f0-0310-95a4-ce76da25c4cd