Fix XMLTABLE on older libxml2

libxml2 older than 2.9.1 does not have xmlXPathSetContextNode (released
in 2013, so reasonable platforms have trouble).  That function is fairly
trivial, so I have inlined it in the one added caller.  This passes
tests on my machine; let's see what the buildfarm thinks about it.

Per joint complaint from Tom Lane and buildfarm.