- 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...)

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

Fix typos, path to file which was moved and renamed and replace "which mdadm" call to file_exist('/proc/mdstat') as we are not using mdadm in this script anyway.

Implemented: FS#1252 - Add permission system based on user / password to app installer 

Fixed: FS#1229 - ISPConfig3 creates the desired e-mail domain twice (on master server AND on the selected Slave)