(ok 3.2.0) Config directive for default charset on import

To continue with what has been said on this page: https://sourceforge.net/tracker/index.php?func=detail&aid=2080963&group_id=23067&atid=377408

In harmony with $cfg['Export']['charset'], that set default charset on the Export page, let's make another one - $cfg['Import']['charset'], that will set default charset on the Import page.

libraries/display.import.lib.php line 91:
if ($temp_charset == $charset) {

Replace with:
if ((empty($cfg['Import']['charset']) && $temp_charset == $charset)
|| $temp_charset == $cfg['Import']['charset']) {

libraries/config.default.php, "Import defaults" block.
Add empty directive:
$cfg['Import']['charset'] = '';

Documentation.txt, line 1044:
Setting this to TRUE also activates a pull-down menu in the Export page, to choose the character set when exporting a file. The default value in this menu comes from $cfg['Export']['charset'].

As variant:
Setting this to TRUE also activates a pull-down menu in the Export and Import page, to choose the character set when exporting or importing a file. The default value in this menu comes from $cfg['Export']['charset'] and $cfg['Import']['charset'], respectively.