Fix handling of duplicate port forwardings; they were effectively cancelling

Fix handling of duplicate port forwardings; they were effectively cancelling
out, but are now just ignored.
(We should make more effort to prevent duplicates before they get as far as
ssh_setup_portfwd() -- it's currently trivially easy to enter them in the
GUI and on the command line, let alone both -- but there's bound to be someone
with a saved session containing dupes out there by now, and anyway there are
duplicates we can't detect before getting this far, for instance
"1234:localhost:22" vs "1234:localhost:ssh".)

[originally from svn r8623]