network: Fetch based browser networking

[basicer]

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

Somewhat fixes #198

Just enough networking for http to work for package managers and net-boot. The networking stack just reflects the mac address of packets it receives, so shouldn't 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