Hey Guix!

A few months ago I mentioned the idea of adding the ability to 
have services automatically restarted when running "guix system 
reconfigure". These patches are a start on making that happen. 
They're incomplete (in particular, documentation is missing), but 
I'm offering them up for comment.

The broad idea is to add a new field to our guix shepherd 
services: restart-strategy. There are three valid values:

- always: this service is always safe to restart when running 
  reconfigure
 
- manual: this service may not be safe to restart when running 
  reconfigure - a message will be printed telling the user to 
  restart the service manually, or they can provide the 
  --restart-services flag to reconfigure to automatically restart 
  them

- never: this service is never safe to restart when running 
  reconfigure (eg. udev)

I have added the flag to the guix daemon's shepherd service to 
show how it works. I tested this by changing my substitute servers 
in config.scm, and after running "reconfigure" I saw my updated 
substitute servers in ps without having to run "sudo herd restart 
guix-daemon".

If nobody has any feedback in the next few days then I'll update 
the manual and send through another patch.

Carlo