Emulate a networking stack, intercept HTTP requests, serve
them with fetch().  Enable by setting networking_proxy=fetch

Somewhat fixes copy#198

Just enough networking for http to work for package managers and
net-boot. The networking stack just reflects the macaddress of
packets it receives, so shouldnt be bothered by mac address
changes. Services include:

- ARP replies by establishing the gateways IP
- DHCP server sending gateway, dns, and client-ip
  - Gateway: 192.168.86.1
  - Client: 192.168.86.100
  - Netmask: 255.255.255.0
- UDP DNS server, all addresses resolve to: 192.168.87.1
- NTP Server, giving out the current time from Date()
- Replies to ICMP pings to any IP address
- UDP echo server on port 8

Limitations:
- Request are limited by CORS rules, but CORS-proxies can work around
  this.
- Redirects are handled transparently by fetch. The upside is sites that
  force an http->https upgrade appear to the VM to work over http.

Future Work:
- Allow HTTP requests from browser to be served by VM.
- HTTPS support (maybe with help of a rust based TLS stack in WASM)
- Support for POSTing binary data

- Add code to establishing TCP connection to VM
- Cleanup