cgroup2: use new fstype for unified hierarchy

Since Linux v4.4-rc1, __DEVEL__sane_behavior does not exist anymore and
is replaced by a new fstype "cgroup2".

With this patch, systemd no longer supports the old (unstable) way of
doing unified hierarchy with __DEVEL__sane_behavior and systemd now
requires Linux v4.4 for unified hierarchy.

Non-unified hierarchy is still the default and is unchanged by this
patch.

https://github.com/torvalds/linux/commit/67e9c74b8a873408c27ac9a8e4c1d1c8d72c93ff

src/basic/missing.h

@@ -437,6 +437,10 @@ struct btrfs_ioctl_quota_ctl_args {
 #define CGROUP_SUPER_MAGIC 0x27e0eb
 #endif
 
+#ifndef CGROUP2_SUPER_MAGIC
+#define CGROUP2_SUPER_MAGIC 0x63677270
+#endif
+
 #ifndef TMPFS_MAGIC
 #define TMPFS_MAGIC 0x01021994
 #endif