extunix dependency can be replaced by tuntap

[avsm]

The https://github.com/mirage/ocaml-tuntap exposes getifaddrs that is parsed into an Ipaddr.t already. That would remove Extunix with a smaller library in the form of tuntap in config_parser.ml

[haesbaert]

Yep, we could do that, but I've started rewriting extunix's recvmsg to be able to have multiple ancillary control options, basically I need IP_RECVIF in bsds and PKTINFO in linux. Extunix doesn't use ctypes which makes it kinda painful.

[avsm]

Yeah binding to them directly in a minilibrary with ctypes makes life much easier from a mirage perspective.

[haesbaert]

I can reconsider this, it seemed more logical at first to add to an existing library.

[haesbaert]

The dependency can now go away as I've written my own recvmsg wrapper, so won't rely on my future work on extunix.

[haesbaert]

Replaced extunix with ocaml-tuntap, that pull-request I made is necessary though.