* gnu/system.scm (<operating-system>)[groups]: Change default to just
the 'root' group.
[sudoers]: New field.
(etc-directory): Add #:sudoers parameter. Add 'sudoers' to the
file union.
(operating-system-etc-directory): Pass #:sudoers to 'etc-directory'.
(%setuid-programs): Add 'sudo'.
(%sudoers-specification): New variable.
* gnu/system/linux.scm (base-pam-services): Add 'sudo'.
* build-aux/hydra/demo-os.scm: Add 'groups' field; add 'guest' to the
'wheel' group.
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