'China's Fedex' takes backdoor route to $6.6 bln listing

* SF Holdings in accord with little-known listed metals firm * Asset swap, share deal with Shenzhen-listed Dingtai * 8 bln yuan new shares sale to finance growth * Biggest 'backdoor listing' since Focus Media $7.2 bln deal * China regulators stepping up backdoor deal scrutiny (Add industry details, background, graphic) By Donny Kwok and Twinnie Siu HONG KONG, May 23 (Reuters) - Giant Chinese courier company SF Holdings (Group) has struck a 43.3 billion yuan ($6.6 billion) deal with a little-know