Add DOMXPath::$enableRegisterNodeNS property

Fix for [bug #55700](https://bugs.php.net/bug.php?id=55700).

As this is a small feature with no BC implications, this patch targets 5.6.
There is no leak here (that I can see, checked with a debug build) - note that this is a property accessor and not a method, as such `retval` is a `zval **` and so would be free'd by the caller. Were you reading this as a locally alloc'd and not free'd variable as it would be if this were a method dealing with `return_value`?

I've added two reads to the test to cover the code path it previously did not hit.