pgsql: Improve pg_dump's checkSeek() function to verify the functioning

Log Message:
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Improve pg_dump's checkSeek() function to verify the functioning of ftello
as well as fseeko, and to not assume that fseeko(fp, 0, SEEK_CUR) proves
anything. Also improve some related comments. Per my observation that
the SEEK_CUR test didn't actually work on some platforms, and subsequent
discussion with Robert Haas.

Back-patch to 8.4. In earlier releases it's not that important whether
we get the hasSeek test right, but with parallel restore it matters.

Modified Files:
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pgsql/src/bin/pg_dump:
pg_backup_archiver.c (r1.185 -> r1.186)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_archiver.c?r1=1.185&r2=1.186)
pg_backup_custom.c (r1.45 -> r1.46)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_custom.c?r1=1.45&r2=1.46)