- rename at_addr to atalk_addr, netrange to atalk_netrange
- use kerneldoc style comments for some structs and functions
- move the IP over DDP handling code out of atalk_rcv into handle_ip_over_ddp,
  that is optimized out when this feature was not selected by the user
- move the routing handling code out of atalk_rcv into atalk_route_packets
- create ddp_hdr and aarp_hdr inline functions to access those protocol headers
  on the skb
- lots of CodingStyle cleanups