contrib/xml2 function name change and minor bitrot fix

Hi,

I've done a bit of testing now and it seems that both sets of XML
functions will coexist in the same database (they both load OK and I can
use functions from both without any apparent problem).

I have changed the name of the new parse function to xml_valid and fixed
a reference to SortMem which meant that the code as supplied would work
against 7.3 and 7.4 but wouldn't work in CVS. Oops! Patch is attached.

Regards

John