The `--rootless` flag had a couple of issues:
* moby#38702: euid=0, $USER="root" but no access to cgroup ("rootful" Docker in rootless Docker)
* moby#39009: euid=0 but $USER="docker" (rootful boot2docker)

To fix moby#38702, XDG dirs are ignored as in rootful Docker, unless the
dockerd is directly running under RootlessKit namespaces.

RootlessKit detection is implemented by checking whether `$ROOTLESSKIT_STATE_DIR` is set.

To fix moby#39009, the non-robust `$USER` check is now completely removed.

The entire logic can be illustrated as follows:

```
withRootlessKit := getenv("ROOTLESSKIT_STATE_DIR")
rootlessMode := withRootlessKit || cliFlag("--rootless")
honorXDG := withRootlessKit
useRootlessKitDockerProxy := withRootlessKit
removeCgroupSpec := rootlessMode
adjustOOMScoreAdj := rootlessMode
```

Close moby#39024
Fix moby#38702 moby#39009

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 3518383)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>