Installer: Try to create kbconfig.php if it does not exist.

[hglock]

Currently the first check for a scratch installation fails, if kbconfig.php file does not exist.
(The file is not delivered with the install package, as it would otherwise overwrite a potentially existing file.)
If kbconfig.php is not there, the installer reports an error. One can simply create this file just by using the touch command. Having an empty kbconfig.php is ok for the installer, it will then create its content.
So, installer shall be updated that if the file does not exist, the installer itself shall try to create it by touching the file. Only if also this fails, an error shall be reported.