Fix multi mapping for rootless containers

[Furisto]

This enables multi mapping for rootless containers. You need something like this in your runtime spec:

"uidMappings": [
	{
		"containerID": 0,
		"hostID": 1000,
		"size": 1
	},
	{
		"containerID": 1,
		"hostID": 100000,
		"size": 65536
	}
],
"gidMappings": [
	{
		"containerID": 0,
		"hostID": 1000,
		"size": 1
	},
	{
		"containerID": 1,
		"hostID": 100000,
		"size": 65536
	}
],

Start the container and then (in container or from the host system with the container pid)

cat /proc/self/uid_map  // or gid_map
0       1000          1
1     100000      65536

You also need to ensure that your user is allowed to map these id ranges. You can check that with

cat /etc/subuid // or subgid
furisto:100000:65536

You also need to have shadow-utils installed.

[Furisto]

Will take a look at the test errors tomorrow.