Support more stable guards for live CDs

Since livecd environments don't have persistent storage across sessions, they can't keep guard nodes across, and as such don't get the benefit from them.

This may be fixable. Suppose that the livecd gathers a set of system hardware information (MAC address, PCI stuff, etc), and hashes it into a "Guard Seed". Or the user could run a small program before burning the cd that sets a random seed on the disk. The Tor client could then be configured to pick its guards based on the seed. This would give the user similar guards across invocations, to avoid guard churn.

One (approximate) solution is to pick guard nodes based on the first N nodes sorted by H(Seed|NodeID). This doesn't do weighting correctly, though.

A better design and a Proposal are probably needed. Posting this here so we don't lose track of it entirely.

(rransom helped develop this idea.)