* gnu/system/shadow.scm (<user-account>)[system?]: New field.
* gnu/system.scm (user-account->gexp): Add it.
* guix/build/activation.scm (add-user): Add #:system? parameter and
honor it.
(activate-users+groups): Handle the 'system?' part of user tuples.
Pass it to 'add-user'. Don't create PROFILE-DIR when SYSTEM? is
true.
* gnu/services/dbus.scm (dbus-service): Add 'system?' field for
"messagebus" account.
* gnu/services/base.scm (guix-build-accounts): Likewise.
* gnu/services/avahi.scm (avahi-service): Likewise.
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