pg_dump: Reorganize dumpFunc() and dumpAgg()

Similar to daa9fe8a, instead of
repeating the almost same large query in each version branch, use one
query and add a few columns to the SELECT list depending on the
version.  This saves a lot of duplication.
Reviewed-by: Fabien COELHO <coelho@cri.ensmp.fr>
Discussion: https://www.postgresql.org/message-id/flat/6594334b-40fd-14f1-6bc5-877afa3feed5@2ndquadrant.com