* gnu/system/shadow.scm (guix-build-accounts): Move to...
* gnu/system/dmd.scm (guix-build-accounts): ... here.
(<service>)[user-accounts, user-groups]: New fields.
(guix-service): New #:build-user-id and #:build-accounts parameters.
Use 'guix-build-accounts' and set the 'user-accounts' and
'user-groups' fields accordingly.
* gnu/system/vm.scm (system-qemu-image): Remove use of
'guix-build-accounts'. Augment ACCOUNTS and GROUPS from what SERVICES
demand.
* gnu/system/shadow.scm (guix-build-accounts): New procedure.
* gnu/system/vm.scm (system-qemu-image): Use it. Add the "guixbuild"
group.
* gnu/system/dmd.scm (guix-service): Add 'builder-group' parameter.
Pass 'guix-daemon' the '--build-users-group' option.