pgsql: Make casts from xml to text independent of the XML option

Log Message:
-----------
Make casts from xml to text independent of the XML option setting, thus
immutable and indexable.  Also fix the volatility settings of some other
XML-related functions.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        xml.c (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.60&r2=1.61)
    pgsql/src/include/catalog:
        catversion.h (r1.437 -> r1.438)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.437&r2=1.438)
        pg_cast.h (r1.36 -> r1.37)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_cast.h?r1=1.36&r2=1.37)
        pg_proc.h (r1.478 -> r1.479)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.478&r2=1.479)
    pgsql/src/test/regress/expected:
        opr_sanity.out (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/opr_sanity.out?r1=1.78&r2=1.79)
    pgsql/src/test/regress/sql:
        opr_sanity.sql (r1.63 -> r1.64)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/opr_sanity.sql?r1=1.63&r2=1.64)