Hey,

I use Shepherd to manage my user session, and if I log out then 
Shepherd leaves all my services running. This patch handles 
SIGTERM and SIGHUP to prevent that.

I hope the guix-patches mailing list was the right place to send 
it. From what I understand Shepherd development is done through 
the guix lists, so it seemed to make the most sense to me.

It's also worth noting that I had to run `gettextize` to get 
Shepherd to build on current master, but I haven't included those 
changes in this commit (because I don't know what should be 
committed and what shouldn't).

Carlo