guardmgr: Use a better persistent data format

Previously we stored only one guard sample, in a state file called "default_guards". That's not future-proof, since we want to have multiple samples in the future. (guard-spec.txt specifies separate samples for highly restrictive filters, and for bridge usage.)

This patch changes our behavior so that we can store multiple samples in a new "guards" file.

I had thought about automatically migrating from the previous file format and location, but I don't think that's necessary given our current (lack of) stability guarantees.

Closes #176 (closed).