system: Define '%base-user-accounts'.

* gnu/system/shadow.scm (%base-user-accounts): New variable.
* gnu/system.scm (<operating-system>)[users]: Use it as the default value.
* gnu/system/examples/bare-bones.tmpl (users): Use it.
* gnu/system/examples/desktop.tmpl (users): Likewise.
* doc/guix.texi (operating-system Reference, User Accounts): Adjust
  accordingly.