New Kill command

What does this Merge Request do and why?

This MR introduces the gdk kill command to assist in killing off stray runsv processes which can remain after an unclean gdk update or unclean GDK service exit:

$ gdk kill
⚠️  WARNING: You're about to forcibly kill the following runsv processes:

  501 65274     1   0  3:12pm ??         0:00.17 runsvdir -P /Users/ash/src/gitlab/gdks/gdk1/services log: ...........................................................................................................................................................................................................................................................................................................................................................................................................
  501 66153 65274   0  3:14pm ??         0:00.02 runsv praefect-gitaly-0
  501 66154 65274   0  3:14pm ??         0:00.02 runsv postgresql
  501 66155 65274   0  3:14pm ??         0:00.02 runsv rails-web
  501 66156 65274   0  3:14pm ??         0:00.03 runsv gitlab-workhorse
  501 66157 65274   0  3:14pm ??         0:00.02 runsv redis
  501 66158 65274   0  3:14pm ??         0:00.02 runsv sshd
  501 66159 65274   0  3:14pm ??         0:00.01 runsv webpack
  501 66161 65274   0  3:14pm ??         0:00.01 runsv praefect
  501 66164 65274   0  3:14pm ??         0:00.01 runsv rails-background-jobs

ℹ️  This command will stop all your GDK instances and any other process started by runit-based. Check the list above to avoid unwanted results.

Are you sure? [y/N]: y
ℹ️  Running 'gdk stop' to be sure..
ℹ️  Giving runsv processes 5 seconds to die..
✅️ All 'runsv' processes have been killed.

$ gdk kill
ℹ️  No runsv processes detected.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1300 (closed)