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.
Extract `ReadBinaryFile()` and `WriteBinaryFile()` from `torcontrol.cpp`
to its own `readwritefile.{h,cpp}` files, so that it can be reused from
other modules.