Because PHP can be configured without `--filter`, it is not 100% safe to use `filter_var()`. This is problematic for casting `"false"` to `false`, as PHP always casts it to `true`. `FILTER_VALIDATE_BOOLEAN` fixes this, but it may not be available.

Add a new function, `wp_validate_boolean()`, to replace `filter_var( $var, FILTER_VALIDATE_BOOLEAN )`.

Fixes #28170.


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git-svn-id: http://core.svn.wordpress.org/trunk@28368 1a063a9b-81f0-0310-95a4-ce76da25c4cd