Fix for bug #3906 "default country".

In htdocs/lib/common.inc.php, getUserCountry():
- added "global" declaration of "$usr", which was missing
- using "GB" as the fallback country code instead of "EN", which is not a valid country code

In htdocs/newcache.php:
- using "getUserCountry()" to initialize "$sel_country" if no "_POST['country']" was given, instead of "$default_country"
- using "$sel_country" to preselect the country in the countries combobox, instead of "getUserCountrs()"

htdocs/lib/common.inc.php

@@ -1198,7 +1198,7 @@ function __autoload($class_name)
 
 	function getUserCountry()
 	{
-		global $opt, $cookie;
+		global $opt, $cookie, $usr;
 
 		// language specified in cookie?
 		if ($cookie->is_set('usercountry'))
@@ -1223,7 +1223,8 @@ function __autoload($class_name)
 		// default country of installation (or domain)
 		if (isset($opt['template']['default']['country']))
 			return $opt['template']['default']['country'];
-
-		return 'EN';
+		
+		// country could not be determined by the above checks -> return "GB"
+		return 'GB';
 	}
-?>
+?>