pgsql: Replace xmlroot with a properly functioning version that parses

Log Message:
-----------
Replace xmlroot with a properly functioning version that parses the value,
sets the items, and serializes the value back (rather than adding an
arbitrary number of XML preambles as before).

The libxml memory management via palloc had to be disabled because it
crashes when libxml tries to access memory that was helpfully freed
earlier by PostgreSQL.  This needs further thought.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        xml.c (r1.10 -> r1.11)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.10&r2=1.11)
    pgsql/src/test/regress/expected:
        xml.out (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.2&r2=1.3)
        xml_1.out (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out.diff?r1=1.3&r2=1.4)
    pgsql/src/test/regress/sql:
        xml.sql (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql.diff?r1=1.2&r2=1.3)