RFC 4245 section 7.1 specifies the meaning of the "address to bind"
parameter in a "tcpip-forward" request. "0.0.0.0" and "127.0.0.1" are
specified to be all interfaces and the loopback interface respectively
in IPv4, while "" and "localhost" are the address-family-agnostic
equivalents. Switch PuTTY to using the latter, since it doesn't seem
right to force IPv4.
There's an argument that PuTTY should provide a means of configuring the
address family used for remote forwardings like it does for local ones.