RFR: 8087283 : Add support for the XML Signature here() function to the JDK XPath implementation

[Sean Mullan]

Please review this change to add support for the XML Signature here() 
function to the JDK XPath implementation. The here function is not 
defined in its own namespace. Therefore it needs to be implemented by 
extending the function library of XPath in the same namespace.

Currently, the JDK XML Signature implementation uses reflection to 
invoke a method of an implementation-specific class in the JDK XPath 
library to register the here function. This fix provides a much cleaner 
solution that avoids use of reflection by extending the built-in set of 
XPath functions to include the here function.

webrev: http://cr.openjdk.java.net/~mullan/webrevs/8087283/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8087283

Thanks,
Sean