RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.nix.sk
RewriteRule (.*) http://nix.sk/$1 [QSA,L,R=301]
RewriteRule ^([A-Za-z0-9-]+)$ index.php?url=$1 [L]

RewriteCond %{HTTP_HOST} ^(www.)?([A-Za-z0-9-]+).nix.sk
RewriteRule ^(.*)$ index.php?url=%2 [QSA]