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.

[originally from svn r2344]