Since the kernel library no longer depends on the system file, move it
to the common library instead in accordance to the diagram in
doc/design/libraries.md.
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 are currently empty structs but they will be used to pass some
function and object pointers from bitcoin application code to IPC hooks
that run, for example, when a remote object is created or destroyed, or
a new process is created.
