Before this change, if you mounted the root of the smb then it would
give an error on rclone about and periodically in the mount logs:
Statfs failed: bucket or container name is needed in remote
This fix makes the smb backend return empty usage in this case which
will stop the errors and show the default 1P of free space.
See: https://forum.rclone.org/t/error-statfs-failed-bucket-or-container-name-is-needed-in-remote/39631
This introduces a new fs.Option flag, Sensitive and uses this along
with IsPassword to redact the info in the config file for support
purposes.
It adds this flag into backends where appropriate. It was necessary to
add oauthutil.SharedOptions to some backends as they were missing
them.
Fixes#5209
