Before this change there was some ambiguity about whether passwords
were obscured on not passing them into config create or config update.
This change adds the --obscure and --no-obscure flags to make the
intent clear.
It also updates the remote control and the tests.
Fixes#3728
- Change rclone/fs interfaces to accept context.Context
- Update interface implementations to use context.Context
- Change top level usage to propagate context to lover level functions
Context propagation is needed for stopping transfers and passing other
request-scoped values.
