Tuntap integration, to pull after my other PRs

Partial integration, since I did not put any of the MacOS specific code for the PCAP stuff into tuntap.

For now I did not change any of the functionality/interface of mirage-platform, as it is only the first step.

Second step will be to not call the Tuntap functions from mirage-platform, but to have mirari to it instead, with the passing fd feature.