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.



git-svn-id: https://develop.svn.wordpress.org/trunk@28542 602fd350-edb4-49c9-b593-d223f7449a82