- replace double-quotes with single-quotes whenever appropriate

- reword some output
- fix typos

- replaced "exec('/etc/init.d/" calls with "exec($conf['init_scripts'] . '/' ."
- fixed output wording (typos and more...)

remove exec() calls, replace /usr/local/ispconfig with variable, replace double-quoted strings with single-quoted whenever appropriate

Fixed: FS#1285 - Support messages are not listed for the client under Help > Support > View messages 

Fixed: FS#1269 - Field "Max. type of cron jobs (chrooted and full implies url)" in Client limits is misleading 

Fixed: FS#1275 - Port 8081 (apps vhost) should be included in the default open TCP ports in the firewall 

Fixed: FS#1313 - Email > Global Filters > Postfix Whitelist > Add new Whitelist record: "Witelist Address" typo 

Fixed a small bug in the nav.php which tries to lookup a custom module config file even if no module is selected and  enabled the dashboard module for the admin user by default after installation.