Rigorously check integrity of I/O stream data

Even if fwrite() succeeds the data is not necessarily out of the clib's buffers
and writing it eventually could fail. Even if the data is flushed out (explicitly by
fflush() or implicitly by fclose()) the kernel might still hold a buffer.

Previously we have ignored this to a large extent - even in important cases
like writing the flash contents to a file. The results can be truncated
images that would brick the respective machine if written back as is (though
flashrom would not allow that due to a size mismatch). flashrom would not
indicate the problem in any output - so far we only check the return value
of fwrite() that is not conclusive.

This patch checks the return values of all related system calls like fclose()
unless we only read the file and are not really interested in output errors.
In the latter case the return value is casted to void to document this fact.
Additionally, this patch explicitly calls fflush() and fsync() (on regular files only)
to do the best we can to guarantee the read image reaches the disk safely
and at least inform the user if it did not work.

Corresponding to flashrom svn r1902.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Urja Rannikko <urjaman@gmail.com>

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