improved crojob

Fixed cron to update serial and not sign directly. This now issues a job for the cron server which will then generate the zone and sign it.
cleaner solution AND fixes a possible replication (zone-transfer) issue.

- added cron for letsencrypt renewal

fixed error in mail_backup -> all mail-backups would have been deleted with old code because of * in filename

Conflicts:
	interface/lib/classes/remoting.inc.php
	interface/web/mail/templates/mail_user_backup_list.htm
	interface/web/sites/lib/lang/en_web_backup_list.lng
	interface/web/sites/lib/lang/es_web_backup_list.lng
	interface/web/sites/lib/lang/tr_web_backup_list.lng
	server/plugins-available/backup_plugin.inc.php

Fix by dirkd: Since file() doesn't remove "\n" from the lines read the condition $lines[$l] == '' never evaluated to true. So $mailHeaders would always contain every line from the template while $mailBody contained none of them.

Together with the bugfix of PHP#68776 [1] disallowing multiple "\n"'s & preceding "\n" in mail()'s $additionalHeaders arg this caused mails not being sent because $mailHeaders would include at least one "\n\n" which was meant to separate the mail headers from the body.

[1] https://bugs.php.net/bug.php?id=68776