pgsql: Fix xmlelement() to initialize libxml correctly before using it,

Log Message:
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Fix xmlelement() to initialize libxml correctly before using it, and to avoid
assuming that evaluation of its input expressions won't change the state of
libxml. This requires refactoring xml_init() to not call xmlInitParser(),
since now not all of its callers want that. I also tweaked things to avoid
repeated execution of one-time-only tests inside xml_init(), though this is
mostly for clarity rather than in hopes of saving any noticeable amount of
runtime. Per report from Sheikh Amjad and subsequent discussion.
In passing, fix a couple of inadequately schema-qualified queries.

Modified Files:
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pgsql/src/backend/utils/adt:
xml.c (r1.49 -> r1.50)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.49&r2=1.50)