Many improvements for ncm-accounts

* Configure`/etc/login.defs`.
* Fixes for accounts coming from NIS
* More flexible behaviour on account removal:
 * Remove any accounts not present in the profile or in the `kept_*` lists.
  * Remove accounts not present in the profile or in the `kept_*` lists and below an UID (non-system accounts, that is)
  * Do not remove any accounts.