This affects the cache_dir and the as-yet-unused state_dir.  It uses
the shellexpand and directories crates so that the default values
can be constant strings that use variables to refer to the right
default locations.

This commit adds configuration options for these values, with the
right defaults, and uses those options instead of built-in functions
to set them.

We also remove the function to extract information from chutney
directories: now that arti is configurable, it can be chutney's job
to make its own network configurations.

This crate is mainly intended to wrap the config.rs code if
necessary, and to give a way for interacting with it on the command
line.

The documentation and implementation diverged; the documentation was
correct.

With this patch we don't consider a consensus to be even potentially
well-signed if the authorities that are listed as signing it don't
contain enough authorities we believe in.  Otherwise, we'd just try
fetching certs for them and failing forever.

(I found this by switching from a chutney network to the main
network without cleaning out my cache.)

Closes #44

We should find out if we believe in the authorities before we even
think of downloading their certs.

Also replace some "disk" with "cache" in log messages

Based rust nightly cargo fmt.