pgsql: Allow XML fragment to contain a XML declaration.

Log Message:
-----------
Allow XML fragment to contain a XML declaration. For that, we need a small
hand-crafted parser for the XML declaration, because libxml doesn't seem
to allow this.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.11&r2=1.12)