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.
DecopeAsmap is a pure utility function and doesn't have any
dependencies on addrman, so move it to util/asmap.
Reviewer hint: use:
`git diff --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space`
