This change checks the context whenever rclone might retry, and
doesn't retry if the current context has an error.
This fixes the pathological behaviour of `--max-duration` refusing to
exit because all the context deadline exceeded errors were being
retried.
This unfortunately meant changing the shouldRetry logic in every
backend and doing a lot of context propagation.
See: https://forum.rclone.org/t/add-flag-to-exit-immediately-when-max-duration-reached/22723
Dircache was changed to:
- Remove special cases for the root directory
- Remove Fatal errors
- Call FindRoot on behalf of the user wherever possible
- Bring up to modern Go standards
Backends were changed to:
- Remove calls to FindRoot
- Change calls to FindRootAndPath to FindPath
- Don't make special cases for the root
This fixes several corner cases, for example removing a non existent
directory if FindRoot hasn't been called.