On some platforms, fsync() returns EBADFD when opening a file descriptor
with O_RDONLY (read-only), leading ultimately now to a PANIC to prevent
data corruption.

This commit adds a new sanity check in pg_fsync() based on fcntl() to
make sure that we don't repeat again mistakes with incorrectly-set file
descriptors so as problems are detected at an early stage.  Without
that, such errors could only be detected after running Postgres on a
specific supported platform for the culprit code path, which could take
some time before being found.  b8e19b93 was a fix for such a problem,
which got undetected for more than 5 years, and a586cc4b fixed another
similar issue.

Note that the new check added works as well when fsync=off is
configured, so as all regression tests would detect problems as long as
assertions are enabled.  fcntl() being not available on Windows, the
new checks do not happen there.

Author: Michael Paquier
Reviewed-by: Mark Dilger
Discussion: https://postgr.es/m/20191009062640.GB21379@paquier.xyz