reflect.SliceHeader is deprecated, however the replacement gives a go
vet warning so this disables the lint warning in one use of
reflect.SliceHeader and replaces it in the other.
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.
