Implement `portfwd-loopback-choice'. Works on local side in Unix as

Implement `portfwd-loopback-choice'. Works on local side in Unix as
well, though it's a lot less useful since you still can't bind to
low-numbered ports of odd loopback IPs. Should work in principle for
SSH2 remote forwardings as well as local ones, but OpenSSH seems
unwilling to cooperate.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2344 cda61777-01e9-0310-a592-d414129be87e