Add context propagation to rclone

- 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.

fs/rc/registry.go

@@ -3,6 +3,7 @@
 package rc
 
 import (
+	"context"
 	"sort"
 	"strings"
 	"sync"
@@ -11,7 +12,7 @@ import (
 )
 
 // Func defines a type for a remote control function
-type Func func(in Params) (out Params, err error)
+type Func func(ctx context.Context, in Params) (out Params, err error)
 
 // Call defines info about a remote control function and is used in
 // the Add function to create new entry points.