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.

Tags:
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REL8_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        xml.c (r1.92.2.1 -> r1.92.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.92.2.1&r2=1.92.2.2)
    pgsql/src/include/utils:
        xml.h (r1.28 -> r1.28.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h?r1=1.28&r2=1.28.2.1)