Refactor to eliminate cast-away-const in pg_dump object sort comparator

While reviewing patch [1], I raised a comment about cast-away-const in pg_dump_sort.c. However, the comment was not accepted and the argument was that the nearby code did the same thing. 

I saw Tom recently had a commit [2] that removed some cast-away-const in ecpg, so I am filing this patch to eliminate all cast-away-const problems in pg_dump_sort.c.

[1] https://postgr.es/m/CALDaNm2x3rd7C0_HjUpJFbxpAqXgm=QtoKfkEWDVA8h+JFpa_w@mail.gmail.com

[2] https://git.postgresql.org/cgit/postgresql.git/commit/?id=4eda42e8bdf5bd3bf69576d54a45c10e7cbc3b35

Best regards,