Adding xpath_exists function

Hi hackers,

Although this is a very small change I figured I'd practice the policy
of outlining your change before you write the code and attempt a patch
submission. Essentially I see the function as a convenience function
that exposes the results of the xpath built in exists() function as a
boolean for easier SQL. The syntax will match the xpath function already
present:

xpath_exists(xpath, xml[, nsarray])

The implementation will check that the xpath value contains
'exists(.....)' and add it if not present for added usability. I can't
blindly wrap the value with exists(...) as exists(exists(.....)) will
always return true in xpath. I've not allocated the oid yet, but will
try to earn my bonus points for getting it close to xpath()'s oid. :)
Appropriate regression tests will be added after I've ensured that 'make
check' is happy I've not regressed anything.

Can I confirm that contrib/xml2 is deprecated and I should be carrying
out my work in backend/utils/adt/xml.c?

I shall be coding it up over the next day or two, work & family permitting!

Thanks,

--
Mike Fowler
Registered Linux user: 379787