Do less without dnixd

[grahamc]

Closes #144

Commit one moves all the "no-dnixd" code into the branch where we don't use it.

Commit two stops writing to the dnixd state directory when dnixd isn't available.

[grahamc]

Updates:

1. Supports reading a token from a file, so I can test this on a machine I don't have copy-paste access to.
2. Adds a ton of wrap_err calls so IO's get contextualized
3. Handles missing nix.conf's when reading existing config files, defaulting to a "".

With these changes, fh login works as expected on a fresh install of vanilla Nix.