Before this change `rclone rc --loopback` would give the error "bad
JSON".
This was because the output of the `rc/list` command was not serialzed
through JSON.
This serializes it through JSON and fixes that (and probably other)
command.
- 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.
This implements a remote control protocol activated with the --rc flag
and a new command `rclone rc` to use that interface.
Still to do
* docs - need finishing
* tests
