Finish implementing & debugging globbing for both FS & libguestfs API.
Refactor and consolidate file reads across OS & libguestfs API.

- Add -d debug command-line option.
- Ignore Emacs auto-save files when loading files from module dir.
- Handle empty files & missing directories.
- Make eth_device validator no longer care about OS/API usage.
PEP 8.
Make omit_mountpoint default to False in find_files().