pgsql: Mapping schemas and databases to XML and XML Schema.

Log Message:
-----------
Mapping schemas and databases to XML and XML Schema.

Refactor and document the remaining mapping code.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.371 -> r1.372)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.371&r2=1.372)
    pgsql/src/backend/utils/adt:
        xml.c (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.37&r2=1.38)
    pgsql/src/include/catalog:
        pg_proc.h (r1.452 -> r1.453)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.452&r2=1.453)
    pgsql/src/include/utils:
        xml.h (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h.diff?r1=1.17&r2=1.18)