(bug 15673) Add indentation and newlines to format=wddxfm output:

(bug 15673) Add indentation and newlines to format=wddxfm output:
* Like the JSON formatter, use our own WDDX formatter when PHP's is not available or the user asked for pretty output (PHP's doesn't do indentation)
* Don't prettify when our own formatter is used because PHP's is not avaiable
* Detect regular arrays (with sequential integer keys) and use <array> for them, rather than using <struct> for all arrays. PHP's formatter does this as well. This change should make our WDDX formatter's output identical to PHP's