WIP: tuntap creation and reading

[si74]

NOTE: The runCmd, interface creation, etc will be merged into forked tuntap package at a later point. The remainder of this PR (ARP-related code, has been merged into main).

TODOs:

• clean up cruft on macb w/ old routes and tun iface [X] - abandoned
• ~tun/tap basic reading [X]~
• ~confirm tun/tap writing works [X]~
• ~ensure addrs/routes added correctly[X] - having trouble here~
• CLEANUP: modularize interface creation/adding addrs [X]
• have tun/tap ipaddr route code work for openbsd, windows, linux [ ]
• ~more efficient data-writing (allocate buffer for an entire frame and properly read input data, then append ARP header) [X]~
• ~ARP stuffs - marshal/unmarshal, internal arp cache, send ARP request [X]~
• open tcp conn to controller and read/receive data [ ]
  • handle ingress (read from tap/tun -> send via tcp conn)
  • handle egress (read from tcp/conn -> write to tap/tun)

[si74]

Note: Right now we're shelling out an iproute2 command for route-setting. But discovered that we can use rtnetlink and the netlink sockets to perhaps achieve some of this work.

Take a look at this: https://man7.org/linux/man-pages/man7/netlink.7.html

And at this: https://github.com/vishvananda/netlink

[si74]

@nicboul as per our chat:

Brainstorming doc here: https://docs.google.com/drawings/d/1roLmnkihKfqFOXS_ZxdSqFfvWaGotA8RPmPBtRcBsck/edit

1. I'll create an arp.go with general functions in it
2. will finish back functionality relating to sending arp, waiting for response, etc
3. reconstitute code from this WIP branch into main PR
4. then we work together on ARP table cleanup

[si74]

this code has been integrated into main go client so closing this