Allow to add more than one group for service user

[Reza Alizadeh Majd]

Hi Guix, 

coming from this thread: 
https://lists.gnu.org/archive/html/help-guix/2020-07/msg00088.html

I had defined a service that needs to run by a specific user. regarding
application creates a unix domain socket during start and change the
ownership of socket file to specific group.

the problem is that daemon user (the user that runs the service) only
detects it's primary group and don't have permission to `chown` the
socket file. 

I also performed another test and run a bash instance inside a `screen`
using the service. when I check for the user groups, I had seen that
only primary group is detected as group for service user. 

I assume that that this might be a bug on group assignment for service
user. or I might missed something in service definition. 

kind regards,
Reza


-- 
Reza Alizadeh Majd
PantherX Team
https://www.pantherx.org/