[PATCH] Add XMLEXISTS function from the SQL/XML standard (was: Add xpath_exists Function)

I've been reading the SQL/XML standard and discovered that it defines a 
function named XMLEXISTS that does exactly what the todo item 
xpath_exists defines. My original patch named the function as per the 
todo but I think using the function name from the standard is a better 
idea. So this patch is the same as before, but the function is now named 
XMLEXISTS instead of xpath_exists.

Regards,

-- 
Mike Fowler
Registered Linux user: 379787