rootless: fix killing daemon

[AkihiroSuda]

- What I did

Fixed the issue where the daemon kept running even when dockerd-rootless.sh (and its descendent) was killed.

$ dockerd-rootless.sh --experimental &
$ ps
   PID TTY          TIME CMD
  1083 pts/1    00:00:00 bash
  8696 pts/1    00:00:00 dockerd-rootles
  8702 pts/1    00:00:00 rootlesskit
  8706 pts/1    00:00:00 exe
  8717 pts/1    00:00:00 slirp4netns
  8733 pts/1    00:00:00 dockerd-rootles
  8740 pts/1    00:00:00 dockerd
  8886 pts/1    00:00:00 ps
$ kill 8696
$ ps
   PID TTY          TIME CMD
  1083 pts/1    00:00:00 bash
  8702 pts/1    00:00:00 rootlesskit
  8706 pts/1    00:00:00 exe
  8717 pts/1    00:00:00 slirp4netns
  8733 pts/1    00:00:00 dockerd-rootles
  8740 pts/1    00:00:00 dockerd
  8887 pts/1    00:00:00 ps
$ kill 8702
$ ps
   PID TTY          TIME CMD
  1083 pts/1    00:00:00 bash
  8706 pts/1    00:00:00 exe
  8733 pts/1    00:00:00 dockerd-rootles
  8740 pts/1    00:00:00 dockerd
  8888 pts/1    00:00:00 ps
$ kill 8706
$ ps
   PID TTY          TIME CMD
  1083 pts/1    00:00:00 bash
  8740 pts/1    00:00:00 dockerd
  8889 pts/1    00:00:00 ps

The causes of the issue were:

1. The parent process of RootlessKit had invoked the child process with Pdeathsig, but it was invalidated after running newuidmap (Support new pid namespace or ability to kill all children rootless-containers/rootlesskit#65)
2. dockerd-rootless.sh had invoked rootlesskit and dockerd without setting up trap for propagating signals

- How I did it

1. updated RootlessKit to v0.4.1 with fix child: reconfigure Pdeathsig (release v0.4.1) rootless-containers/rootlesskit#66 (full changes: rootless-containers/rootlesskit@e92d5e7...27a0c7a)
2. updated dockerd-rootless.sh to exec the processes without forking

- How to verify it

$ dockerd-rootless.sh --experimental &
$ ps
   PID TTY          TIME CMD
  1083 pts/1    00:00:00 bash
  8460 pts/1    00:00:00 rootlesskit
  8470 pts/1    00:00:00 exe
  8484 pts/1    00:00:00 slirp4netns
  8500 pts/1    00:00:00 dockerd
  8653 pts/1    00:00:00 ps
$ kill 8460
$ ps
   PID TTY          TIME CMD
  1083 pts/1    00:00:00 bash
  8654 pts/1    00:00:00 ps

- Description for the changelog

rootless: fix killing daemon

- A picture of a cute animal (not mandatory but encouraged)
https://twitter.com/13033303/status/1128454769050128384

[AkihiroSuda]

@tonistiigi @tiborvass

[cpuguy83]

LGTM