pcap support

[zestrada]

This PR adds pcap support for traffic over vpnguin. In an attempt to reduce guest resource demand, we synthesize packets received/transmitted over vsock from the host rather than leaning on the guest to capture traffic. TCP, UDP, IPv4, and IPv6 are supported.

Some thoughts:

• We make a fake TCP stream by doing the handshake and then managing sequence numbers. This makes wireshark analyses happier, but anything other than the payload is totally fake.
  • Technically some code existed for teardown, but vpnguin tends to just leave things open so it is untested.
• We use bogus MAC addresses. We could grab the guest MAC address for a given IP by introducing a new HostRequest for doing that if desired. Unclear if that is useful since we define static MAC addresses to make direction of transmission (e.g., host->guest) clear.
• The host VPN is currently torn down by SIGTERM, which currently don't do anything with. I didn't think it was worth tightening up guarantees around whether packets are committed to disk before existing, but we can do that if desired
• An effort was made to be consistent with support specifying source address #19 for the source IP address

[zestrada]

One other change: since we now require even more libraries for the host, we don't build the host for every architecture, only x86_64 and aarch64.

[zestrada]

Updated to define static MAC addresses for the host and guest. Using 47:55:45:53:54:00 "GUEST" for the guest and 48:4f:53:54:00:00 "HOST" for the host. Note that all IPs will share those MACs, but now it's straightforward to see which direction each packet was sent (by default everything is 127.0.0.1<=>127.0.0.1, so previously could do stuff like look at ephemeral vs. non-ephemeral for known services).

[zestrada]

Squashed the pre Packetbuilder stuff into one commit just so history is there for anyone that might find it interesting