pgsql: Fix xmlconcat by properly merging the XML declarations.

Log Message:
-----------
Fix xmlconcat by properly merging the XML declarations.  Add aggregate
function xmlagg.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execQual.c (r1.207 -> r1.208)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c.diff?r1=1.207&r2=1.208)
    pgsql/src/backend/utils/adt:
        xml.c (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.19&r2=1.20)
    pgsql/src/include/catalog:
        catversion.h (r1.372 -> r1.373)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.372&r2=1.373)
        pg_aggregate.h (r1.59 -> r1.60)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_aggregate.h.diff?r1=1.59&r2=1.60)
        pg_proc.h (r1.437 -> r1.438)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.437&r2=1.438)
    pgsql/src/include/utils:
        xml.h (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h.diff?r1=1.11&r2=1.12)
    pgsql/src/test/regress/expected:
        xml.out (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.9&r2=1.10)
        xml_1.out (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out.diff?r1=1.9&r2=1.10)
    pgsql/src/test/regress/sql:
        xml.sql (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql.diff?r1=1.8&r2=1.9)