Magic incantation to mostly fix MacOS 10.6.

It comes down to this:
   sysctl_set('net.inet.ip.scopedroute', 0)

I say "mostly" because actually it doesn't fix it; sshuttle doesn't know
what to do with the received connection, so there must be a minor bug
remaining somewhere.  I'll fix that next.

Thanks to dkf <dfortunato@gmail.com> on the sshuttle mailing list for
suggesting the magic fix.  He points at this post in particular:
  http://discussions.apple.com/thread.jspa?messageID=11558355&#11558355
that gave him the necessary clue.

firewall.py

@@ -125,6 +125,7 @@ def do_ipfw(port, subnets):
     if subnets:
         sysctl_set('net.inet.ip.fw.enable', 1)
         sysctl_set('net.inet.ip.forwarding', 1)
+        sysctl_set('net.inet.ip.scopedroute', 0)
 
         ipfw('add', sport, 'accept', 'ip',
              'from', 'any', 'to', 'any', 'established')