Convert user config.xml to ascii

Atm the user config.xml file is in UTF (since previously it stored the name of players which requires special characters). The player information is now stored in a separate file, so the config.xml can be written as ascii now (avoiding problems some people have that they can't see or edit the file)

Migrated-From: https://sourceforge.net/apps/trac/supertuxkart/t/ticket/1154
