Before this change the Features() method would return a different Fs
to that the Features() method was called on if the remote was
instantiated on a file.
The practical effect of this is that optional features, eg `rclone
about` wouldn't work properly when called on a file, and likely this
has been causing low level problems for users of these backends for
ages.
Ideally there would be a test for this, but it turns out that this is
really hard, so instead of that all the backends have been converted
to not copy the Fs and a big warning comment inserted for future
readers.
Fixes#2182