bug#56690: greeter user permissions are not enough to talk with seatd

[Liliana Marie Prikler <liliana.prikler@gmail.com>]

Am Donnerstag, dem 04.08.2022 um 12:45 +0300 schrieb muradm:
> * 56690 - gnu: seatd-service-type: Should use seat group.
> With this change, if seatd-service-type is present in the
> system configuration, "seat" group will be added, and seatd
> will run as root/seat. Group is configurable, but default is 
> "seat".
I made it so that by default the sanitizer is used to turn the string
"seat" into a group and used (ice-9 match), reducing some needless
redundancy.  I also reworded the manual to the best of my ability
following our conversations and adapted the commit message.

> * 56699 - gnu: greetd-service-type: Add greeter-extra-groups 
>   config field.
> With this change, if user wants to use seatd-service-type with
> greeter requiring seatd.sock, he can add "seat" group to
> greeter-extra-groups field.
I fixed some minor issue in the manual and reindented the marionette-
type in the tests, also reworded the commit message.

I didn't get the chance to run the system tests – some timeout causes
the marionette build to fail on my machine – but I verified
independently that at least the seatd socket has the right permissions.
I hope this will be enough for you to get gtkgreet running.

Cheers