resolving user and group ids to names, and listing supplementary group IDs.
One is written in pure Go and parses /etc/passwd and /etc/group. The other
is cgo-based and relies on the standard C library (libc) routines such as
getpwuid_r, getgrnam_r, and getgrouplist.

When cgo is available, and the required routines are implemented in libc
for a particular platform, cgo-based (libc-backed) code is used.

	buf := make([]byte, kind.initialSize())
	for {
		errno := f(buf)
		if errno == 0 {
			return nil
		} else if runtime.GOOS == "aix" && errno+1 == 0 {
			// On AIX getpwuid_r appears to return -1,
			// not ERANGE, on buffer overflow.
		} else if errno != syscall.ERANGE {
			return errno
		}
		newSize := len(buf) * 2
		if !isSizeReasonable(int64(newSize)) {