pgsql: Be more user-friendly about unsupported cases for parallel pg_re

Be more user-friendly about unsupported cases for parallel pg_restore.

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.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/76d8bd8b906ddc27e4aee4b92b3f82e896b7e926

Modified Files
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src/bin/pg_dump/pg_backup_archiver.c |    6 ++++++
src/bin/pg_dump/pg_backup_custom.c   |    9 +++++++--
2 files changed, 13 insertions(+), 2 deletions(-)