`fclose()` is flushing any buffered data to disk, so if it fails then
that could mean that the data was not completely written to disk.
Thus, check if `fclose()` succeeds and only then claim success from
`WriteBinaryFile()`.
If an error occurs and `fread()` returns `0` (nothing was read) then the
code before this patch would have returned "success" with a partially
read contents of the file.
Extract `ReadBinaryFile()` and `WriteBinaryFile()` from `torcontrol.cpp`
to its own `readwritefile.{h,cpp}` files, so that it can be reused from
other modules.
