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.

Added the configuration variable $conf['demo_mode'] in the config.inc.php file. It disables the newly introduced demo mode which limits certain actions in the interface part of ispconfig: the demo mode can be used to run the ispconfig interface part as online demo.

Patch from Filip that enables settting of sys_user.modules and sys_user.startmodule via client_add remote API call

Changed quota setting for courier installations in authmysqlrc.master as ispconfig sets the quota with maildirmake command, so we do not have to specify a sql quota field.

Temporary fix for: FS#1311 - Only first PHPINIDir directive honored per configuration tree - subsequent ones ignored