gnu: greetd-service-type: Add supplementary groups to greeter.

gnu: greetd-service-type: Add supplementary groups to greeter.

* gnu/services/base.scm (<greetd-configuration>)
[greeter-supplementary-groups]: New field.
(%greetd-accounts): Rename to...
(greetd-accounts): ... this.  Convert to a function that takes a config
argument.  Use greeter-supplementary-groups.
(greetd-service-type): Adjust accordingly.
* gnu/tests/desktop.scm (%minimal-services): Add test for
greeter-supplementary-groups.
* doc/guix.texi ("Base Services")[greetd-service-type]: Document
greeter-supplementary-groups.