system: Make accounts and groups at activation time.
* gnu/services/base.scm (guix-build-accounts): Remove #:gid parameter;
add #:group. Remove 'password' and 'gid' fields in 'user-account'
form, and add 'group'.
(guix-service): Remove #:build-user-gid parameter. Remove 'id' field
in 'user-group' form.
* gnu/system.scm (etc-directory): Remove #:groups and #:accounts. No
longer produce files "passwd", "shadow", and "group". Adjust caller
accordingly.
(%root-account): New variable.
(operating-system-accounts): Add 'users' variable. Add %ROOT-ACCOUNT
only of 'operating-system-users' doesn't already contain a root
account.
(user-group->gexp, user-account->gexp): New procedures.
(operating-system-boot-script): Add calls to 'setenv' and
'activate-users+groups' in gexp.
* gnu/system/linux.scm (base-pam-services): Add PAM services for
"user{add,del,mode}" and "group{add,del,mod}".
* gnu/system/shadow.scm (<user-account>)[gid]: Rename to...
[group]: ... this.
[supplementary-groups]: New field.
[uid, password]: Default to #f.
(<user-group>)[id]: Default to #f.
(group-file, passwd-file): Remove.
* gnu/system/vm.scm (operating-system-default-contents)[user-directories]:
Remove. Add "/home" to the directives.
* guix/build/activation.scm (add-group, add-user,
activate-users+groups): New procedures.
ab6a279abb