pgsql: Export xml.c's libxml-error-handling support so that contrib/xml2

Log Message:
-----------
Export xml.c's libxml-error-handling support so that contrib/xml2 can use it
too, instead of duplicating the functionality (badly).

I renamed xml_init to pg_xml_init, because the former seemed just a bit too
generic to be safe as a global symbol. I considered likewise renaming
xml_ereport to pg_xml_ereport, but felt that the reference to ereport probably
made it sufficiently PG-centric already.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.96 -> r1.97)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.96&r2=1.97)
pgsql/src/include/utils:
xml.h (r1.30 -> r1.31)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h?r1=1.30&r2=1.31)