Rename Activations -> Unix_activations

Unfortunately all ocamlfind packages foo, foo.bar, foo.bar.baz
end up in the same directory which is added to the ocaml include
path which means you can't hide modules present in submodules
(like Activations)

Since we're functorising over activations anyway, might as well
solve this with a rename.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>