XPath fixes:

 - Function renamed to "xpath".
 - Function is now strict, per discussion.
 - Return empty array in case when XPath expression detects nothing
   (previously, NULL was returned in such case), per discussion.
 - (bugfix) Work with fragments with prologue: select xpath('/a',
   '<?xml version="1.0"?><a /><b />'); // now XML datum is always wrapped
   with dummy <x>...</x>, XML prologue simply goes away (if any).
 - Some cleanup.

Nikolay Samokhvalov

Some code cleanup and documentation work by myself.