Support for detecting stale services

What does this Merge Request do?

This MR adds gdk doctor support for detecting stale processes. On occasion (reasons are not entirely clear at this stage), some of the GDK services go 'stale', even after running gdk stop and typically manifests itself with some runsv processes still running that have a parent PID of 1:

$ ps -A -o pid,ppid,command | egrep '1 [r]unsv '
95010     1 runsv rails-web
95011     1 runsv rails-actioncable
95017     1 runsv geo-cursor

On my machine I had a few stale processes and here's what gdk doctor produced:

$ gdk doctor

Stale Services
--------------------------------------------------------------------------------
The following GDK services appear to be stale:

rails-web
rails-actioncable

You can try killing them by running:

kill 95010 95011

I ran the kill command above and was able to fix the situation.

Closes: #864 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.