The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.
These files change infrequently, and not much header shuffling is required.
We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.
`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.
