improve the usability of `ipa user-del --preserve` command
`ipa user-del` with `--preserve` option will now process multiple entries and
handle `--continue` option in a manner analogous to `ipa user-del` in normal
mode.
In addition, it is now no longer possible to permanently delete a user by
accidentally running `ipa user-del --preserve` twice.
https://fedorahosted.org/freeipa/ticket/5234
https://fedorahosted.org/freeipa/ticket/5236
Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>