If we are unable to do a parallel restore because the input file is stdin
or is otherwise unseekable, we should complain and fail immediately, not
after having done some of the restore.  Complaining once per thread isn't
so cool either, and the messages should be worded to make it clear this is
an unsupported case not some weird race-condition bug.  Per complaint from
Lonni Friedman.

Back-patch to 8.4, where parallel restore was introduced.