Make --cmd only available to root

For security reason, disable --cmd for non-root users
Some useful stuff in fs module

- fs::join
- fs::is_absolute
- fs::expand
- fs::resolve (readlink)
- fs::is_accessible
- fs::PATH_SEPARATOR
Fix fs::is_dir

Now fs::is_dir(path) works even when the user
has no x permission on that path.
Check permissions on mount paths

The checks are only performed for non-root users.

- Enforce absolute path.
- Check write permission of --tmpfs and --bindfs dests.
- Check read permission of --bindfs src.
Bump to 0.9.7