main: support rootless mode in userns

Running rootless containers in userns is useful for mounting
filesystems (e.g. overlay) with mapped euid 0, but without actual root
privilege.

Usage: (Note that `unshare --mount` requires `--map-root-user`)

  user$ mkdir lower upper work rootfs
  user$ curl http://dl-cdn.alpinelinux.org/alpine/v3.7/releases/x86_64/alpine-minirootfs-3.7.0-x86_64.tar.gz | tar Cxz ./lower || ( true; echo "mknod errors were ignored" )
  user$ unshare --mount --map-root-user
  mappedroot# runc spec --rootless
  mappedroot# sed -i 's/"readonly": true/"readonly": false/g' config.json
  mappedroot# mount -t overlay -o lowerdir=./lower,upperdir=./upper,workdir=./work overlayfs ./rootfs
  mappedroot# runc run foo

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

utils.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strconv"
+	"strings"
 
 	"github.com/opencontainers/runtime-spec/specs-go"
 
@@ -81,3 +83,12 @@ func revisePidFile(context *cli.Context) error {
 	}
 	return context.Set("pid-file", pidFile)
 }
+
+// parseBoolOrAuto returns (nil, nil) if s is empty or "auto"
+func parseBoolOrAuto(s string) (*bool, error) {
+	if s == "" || strings.ToLower(s) == "auto" {
+		return nil, nil
+	}
+	b, err := strconv.ParseBool(s)
+	return &b, err
+}