mirage-net now depends on ocaml-ipaddr, removal of duplicated code and adaptation.
Ported the direct backend to using the Ipaddr.V4.* functions, removing ipv4_* functions from Nettypes.
Ported the socket backend to using Ipaddr.V4.* functions as well.
Ipaddr.V4.blank -> Ipaddr.V4.any.
Updated META.in to include ipaddr.
bump to 0.9.4
Use the `Ipaddr` external library and remove the homebrew equivalent.

This syncs direct with socket.