This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
This adds AddOverrideGetter and GetOverride methods to config map and
uses them in fs.ConfigMap.
This enables us to tell which values have been set and which are just
read from the config file or at their defaults.
This also deletes the unused AddGetters method in configmap.
