Fix parallel pg_restore to handle comments on POST_DATA items correctly.

The previous coding would try to process all SECTION_NONE items in the
initial sequential-restore pass, which failed if they were dependencies of
not-yet-restored items.  Fix by postponing such items into the parallel
processing pass once we have skipped any non-PRE_DATA item.

Back-patch into 9.0; the original parallel-restore coding in 8.4 did not
have this bug, so no need to change it.

Report and diagnosis by Arnd Hannemann.