patch for contrib/xml2

Hello,

Here is the patch for contrib/xml2 (attached), that:
- renames xml_valid() to xml_is_well_formed()
- adds PG_MODULE_MAGIC; at the beginning of xpath.c
- introduces xpath_array() function
- adds some words about that function and caution about libraries to README

I understand that it's too late, but xpath_array() is pretty simple
function (and, actually, this is a tiny step to XQuery sequences).
Besides that, it is mentioned in the current TODO list and I've told
about this before. Actually, this is at least some [minor] addition for
this module for 8.2.

Let me know if I can help somehow else in this direction.

--
Best regards,
Nikolay