Improve pg_dump's checkSeek() function to verify the functioning of ftello

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.