pgsql: Fix machine-dependent crash in sqlchar_to_unicode().

Log Message:
-----------
Fix machine-dependent crash in sqlchar_to_unicode(). Get rid of
bletcherous and unsafe manipulation of global encoding setting.
Clean up libxml reporting mechanism a bit (it still looks like a
dangling-pointer crash waiting to happen, though, not to mention
being far less than sane from a localization standpoint).

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.3&r2=1.4)
pgsql/src/backend/utils/mb:
mbutils.c (r1.60 -> r1.61)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c.diff?r1=1.60&r2=1.61)
pgsql/src/include/mb:
pg_wchar.h (r1.69 -> r1.70)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h.diff?r1=1.69&r2=1.70)