 Function to get and set a string in a system or user specific file. This
 enables, for example, system specific paths to binaries to be saved.

 The specified string will be saved in a file named &lt;string_name&gt;.txt,
 either in a subfolder named .ignore under this file's folder, or in the
 user's prefdir folder (in case this file's folder is non-writable).

 IN:
   string_name - String containing the name of the string required, which
                 sets the filename storing the string: &lt;string_na