Fixes redmine #2988 otrs #1221. Get rid of special chars in debian6/7's /etc/issue

What it does is to get rid of escaped characters like \l, \n or any @X.

According to 'man issue' :
[/etc/issue]  may  contain various  @char and \char sequences, if supported by the getty-type program employed on the system.

This problem is not restricted to Debian 7. Debian 6 and Ubuntus can be affected too.