local-tar-basebackup: ignore pg_dynshmem, pg_snapshot in a common empty dir list
local-tar-basebackup: ignore pg_dynshmem, pg_snapshot in a common empty dir list

These two directories don't contain files that a restored cluster can use,
so just ignore them to avoid possibly bloating the backup with useless data.
Also define the list of empty directories in a single place only.

ohmu/pghoard,hnousiainen/pghoard,saaros/pghoard,ohmu/pghoard,hnousiainen/pghoard