socket: add Net.Manager.get_intfs

This is exposed by the direct backend and used by mirage-skeleton/tcp

Related to mirage/mirage-skeleton#9
socket: Net.Manager.id should be the same as OS.Netif.id

For compatability with the direct backend.

Related to mirage/mirage-skeleton#9
socket: Net.Manager.get_intf_ipv4addr needs to exist

For compatability with the direct backend

Related to mirage/mirage-skeleton#9
Merge pull request #32 from djs55/update-socket-interface

Add missing functions to the socket backend