// a given user or group. getsubids produces a line for each mapping configured.
// Here we expect that there is a single mapping, and the same values are used for the subordinate user and group ID ranges.
// The output is something like:
// $ getsubids kubelet
// 0: kubelet 65536 2147483648
// $ getsubids -g kubelet
// 0: kubelet 65536 2147483648
func parseGetSubIdsOutput(input string) (uint32, uint32, error) {