Add option to halt system instead of process exit

[randalloveson]

The final thing a true init needs to do--or rather not do--that Tini does not support is not exit like a normal process. When running as true init, Tini causes a kernel panic by exiting.

The proposed options make Tini a workable init for a micro VM, instead of just a Linux namespaces container.

[randalloveson]

This PR also includes a commit to update the build to Ubuntu 20.04, which fixes this problem:

  Downloading https://files.pythonhosted.org/packages/96/2f/caec18213f6a67852f6997fb0673ae08d2e93d1b81573edb93ba4ef06970/pip-22.1.2-py3-none-any.whl (2.1MB)
Installing collected packages: pip
  Found existing installation: pip 8.1.1
    Not uninstalling pip at /usr/lib/python3/dist-packages, outside environment /usr
Successfully installed pip-22.1.2
+ python3 -m pip install virtualenv
Traceback (most recent call last):
  File "/usr/lib/python3.5/runpy.py", line 184, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.5/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.5/dist-packages/pip/__main__.py", line 29, in <module>
    from pip._internal.cli.main import main as _main
  File "/usr/local/lib/python3.5/dist-packages/pip/_internal/cli/main.py", line 57
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax
The command '/bin/sh -c /install_deps.sh' returned a non-zero code: 1

[Yaroslav-95]

This would be useful, for example, for using tini with User-Mode Linux when testing Linux Kernel modules: https://www.kernel.org/doc/html/latest/virt/uml/user_mode_linux_howto_v2.html