shadow_audit_result for the result argument of audit_logger().
This permits stronger type checking and a better readability of
the results (SHADOW_AUDIT_FAILURE/SHADOW_AUDIT_SUCCESS constants).
* src/groupadd.c, src/groupdel.c, src/useradd.c, src/userdel.c:
Use the SHADOW_AUDIT_FAILURE/SHADOW_AUDIT_SUCCESS results instead
of 0 or 1 in audit_logger().