runc: add support for rootless containers

This enables the support for the rootless container mode. There are many
restrictions on what rootless containers can do, so many different runC
commands have been disabled:

* runc checkpoint
* runc events
* runc pause
* runc ps
* runc restore
* runc resume
* runc update

The following commands work:

* runc create
* runc delete
* runc exec
* runc kill
* runc list
* runc run
* runc spec
* runc state

In addition, any specification options that imply joining cgroups have
also been disabled. This is due to support for unprivileged subtree
management not being available from Linux upstream.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

exec.go

@@ -90,9 +90,6 @@ following will output a list of processes running in the container:
 		if err := checkArgs(context, 1, minArgs); err != nil {
 			return err
 		}
-		if os.Geteuid() != 0 {
-			return fmt.Errorf("runc should be run as root")
-		}
 		if err := revisePidFile(context); err != nil {
 			return err
 		}