Portability fixes for containerd shim

Update go-runc to master with portability fixes.

Subreaper only exists on Linux, and only Linux runs the shim in a
mount namespace.

With these changes the shim compiles on Darwin, which means the
whole build compiles without errors now.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

vendor/github.com/opencontainers/runc/libcontainer/utils/utils_unix.go

@@ -1,43 +0,0 @@
-// +build !windows
-
-package utils
-
-import (
-	"io/ioutil"
-	"os"
-	"strconv"
-	"syscall"
-)
-
-func CloseExecFrom(minFd int) error {
-	fdList, err := ioutil.ReadDir("/proc/self/fd")
-	if err != nil {
-		return err
-	}
-	for _, fi := range fdList {
-		fd, err := strconv.Atoi(fi.Name())
-		if err != nil {
-			// ignore non-numeric file names
-			continue
-		}
-
-		if fd < minFd {
-			// ignore descriptors lower than our specified minimum
-			continue
-		}
-
-		// intentionally ignore errors from syscall.CloseOnExec
-		syscall.CloseOnExec(fd)
-		// the cases where this might fail are basically file descriptors that have already been closed (including and especially the one that was created when ioutil.ReadDir did the "opendir" syscall)
-	}
-	return nil
-}
-
-// NewSockPair returns a new unix socket pair
-func NewSockPair(name string) (parent *os.File, child *os.File, err error) {
-	fds, err := syscall.Socketpair(syscall.AF_LOCAL, syscall.SOCK_STREAM|syscall.SOCK_CLOEXEC, 0)
-	if err != nil {
-		return nil, nil, err
-	}
-	return os.NewFile(uintptr(fds[1]), name+"-p"), os.NewFile(uintptr(fds[0]), name+"-c"), nil
-}