[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,

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Mike Fowler
Registered Linux user: 379787