Improve 'arc get-config', 'arc set-config' and show more config info with --trace

Summary:
Fixes T4952. Several issues:

  - You review configuration values with `arc set-config --show`. This makes no sense and never has, I think it just predated `arc get-config` or was easier or something.
    - Instead, review values with `arc get-config` and review details with `arc get-config --verbose`.
  - Show better and more detailed information about all config sources.
  - Establish and show default values from a new "default" source.
  - With `--trace` include more information about attempts to read configuration files.

Test Plan:
Ran `arc get-config --trace --verbose` in various working directories and received sensible-looking output.

{F156247}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4952

Differential Revision: https://secure.phabricator.com/D9172