config: make config file reads reload the config file if needed #4996

Before this change the config file needed to be explicitly reloaded.
This coupled the config file implementation with the backends
needlessly.

This change stats the config file to see if it needs to be reloaded on
every config file operation.

This allows us to remove calls to

- config.SaveConfig
- config.GetFresh

Which now makes the the only needed interface to the config file be
that provided by configmap.Map when rclone is not being configured.

This also adds tests for configfile

backend/onedrive/onedrive.go

@@ -302,7 +302,6 @@ func init() {
 
 			m.Set(configDriveID, finalDriveID)
 			m.Set(configDriveType, rootItem.ParentReference.DriveType)
-			config.SaveConfig()
 		},
 		Options: append(oauthutil.SharedOptions, []fs.Option{{
 			Name:    "region",