Fix initdb --sync-only to also sync tablespaces.

630cd144 added initdb --sync-only, for use by pg_upgrade, by just
exposing the existing fsync code. That's wrong, because initdb so far
had absolutely no reason to deal with tablespaces.

Fix --sync-only by additionally explicitly syncing each of the
tablespaces.

Backpatch to 9.3 where --sync-only was introduced.

Abhijit Menon-Sen and Andres Freund