The (gnu system accounts) module is meant to be used both on the build-
and on the host-side.
* gnu/system/shadow.scm <top level>: Call 'default-shell'.
(<user-account>, <user-group>): Move to...
* gnu/system/accounts.scm: ... here. New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add system/accounts.scm.