GetErrorReason()'s Win32 implementation does the same thing as
Win32ErrorString(int err) from syserror.cpp, so call the latter.
Also remove now-unnecessary headers from sock.cpp and less verbose
handling of #ifdefs.
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.