services: Explicitly refer to Shadow when requiring the 'nologin' shell.

* gnu/services/avahi.scm (avahi-service): Change 'shell' to a gexp
  referring to "nologin" in the SHADOW package.
* gnu/services/dbus.scm (dbus-service): Likewise.
* gnu/services/networking.scm (ntp-service, tor-service): Likewise.