system: Add a 'system?' field to user accounts.

* 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.

gnu/services/base.scm

@@ -327,6 +327,7 @@ starting at FIRST-UID, and under GID."
                     (lambda (n)
                       (user-account
                        (name (format #f "guixbuilder~2,'0d" n))
+                       (system? #t)
                        (uid (+ first-uid n -1))
                        (group group)