Hide DNSSEC from Wizard if DNS-Mirrors are available (prevents broken DNSSEC-Implementaion on Multiserver-Setups)

Fixed issue #3955 DNS module: Limit for maximum zones can be circumvented with "import zone file" function.
Removed TODO string in error template.

This Checkbox can be enabled or disabled by template.

Further I fixed an inconsistent design (HTML) which was introduced by @tbrehm yesterday I tink. (DKIM-Checkbox)

- changed/harmonized order of clients in client select fields ("ORDER BY client.company_name, client.contact_name, sys_group.name")

display dns-record for dkim in the mail-domain
rewrite function domain_dkim_update in server/plugins-available/mail_plugin_dkim.inc.php
change dns*.php according to commit 604c0c24

skip writing dkim-values for inactive domains
removed the public-dkim-key from the interface
added german language-file

Fixed/Implemented: replaced intval() by $app->functions->intval() in all interface functions due to big number problem in intval()