Allow remote-to-local forwardings to use IPv6.

Allow remote-to-local forwardings to use IPv6.

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.

[originally from svn r9668]