This function updates the Settings.php file with the changes in config_vars.

• Expects config_vars to be an associative array, with the keys as the variable names in Settings.php, and the values the variable values.
• Does not escape or quote values.
• Preserves case, formatting, and additional options in file.
• Writes nothing if the resulting file would be less than 10 lines in length (sanity check for read lock.)