pgsql: Use fork names instead of numbers in the file names for

Log Message:
-----------
Use fork names instead of numbers in the file names for additional
relation forks. While the file names are not visible to users, for those
that do peek into the data directory, it's nice to have more descriptive
names. Per Greg Stark's suggestion.

Modified Files:
--------------
    pgsql/contrib/pageinspect:
        pageinspect.sql.in (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/pageinspect.sql.in?r1=1.5&r2=1.6)
        rawpage.c (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/rawpage.c?r1=1.7&r2=1.8)
    pgsql/doc/src/sgml:
        pageinspect.sgml (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pageinspect.sgml?r1=1.4&r2=1.5)
        storage.sgml (r1.25 -> r1.26)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml?r1=1.25&r2=1.26)
    pgsql/src/backend/catalog:
        catalog.c (r1.78 -> r1.79)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/catalog.c?r1=1.78&r2=1.79)
    pgsql/src/backend/utils/adt:
        dbsize.c (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/dbsize.c?r1=1.21&r2=1.22)
    pgsql/src/include/catalog:
        catalog.h (r1.41 -> r1.42)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catalog.h?r1=1.41&r2=1.42)
        catversion.h (r1.494 -> r1.495)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.494&r2=1.495)
    pgsql/src/include/storage:
        relfilenode.h (r1.18 -> r1.19)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/relfilenode.h?r1=1.18&r2=1.19)