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

Log Message:
-----------
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:
--------------
    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)