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.
`SEPARATE_ZVAL` would heap-allocate the copy, requiring it to be manually destroyed after the conversion. I'm not necessarily against this, but it does seem a little inefficient. I'm not aware of a macro that would gain us anything here, although I'm happy to be corrected :-)