Log Message:
Updated dtcpclient-20060526.
 - If `@' is specified as an interface for prefix delegation, it is assumed
  as a tunnel interface.
 - Add experimental support for an IPv6 over an IPv4 tunnel using Netgraph.
  The relevant options are -b and -U.
 - Make an IPv6 over an IPv4 tunnel NAT friendly. If -n option is specified,
  it is enabled. It requires some modification to ng_ksocket of server side.
 - Add $up_command and $down_command to run at up/down.
 - Add -B option to specify NAT port.