Don't use fread(), since that doesn't necessarily set errno. We could
use read() instead, but it's even better to use pg_pread(), which
allows us to avoid some extra calls to seek to the desired location in
the file.
Also, advertise a wait event while reading from a file, as we do for
most other places where we're reading data from files.
Patch by me, reviewed by Hamid Akhtar.
Discussion: http://postgr.es/m/CA+TgmobBw-3573vMosGj06r72ajHsYeKtksT_oTxH8XvTL7DxA@mail.gmail.com
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