bug#20646: List of groups/users should be validated

[Ludovic Courtès]

Dave Thompson reported that an ‘operating-system’ declaration where a
user is declared with a ‘group’ that doesn’t exist leads to a failure at
boot-time (‘useradd’ fails.)  This is in GuixSD 0.8.2.

Users/groups should be statically validated when the configuration is
instantiated to avoid this kind of issues.

Ludo’.