Added new --auto-hosts and --seed-hosts options to the client.

Now if you use --auto-hosts (-H), the client will ask the server to spawn a
hostwatcher to add names.  That, in turn, will send names back to the
server, which sends them back to the client, which sends them to the
firewall subprocess, which will write them to /etc/hosts.  Whew!

Only the firewall process can write to /etc/hosts, of course, because only
he's running as root.

Since the name discovery process is kind of slow, we cache the names in
~/.sshuttle.hosts on the remote server.

Right now, most of the names are discovered using nmblookup and smbclient,
as well as by reading the existing entries in /etc/hosts.  What would really
be nice would be to query active directory or mdns somehow... but I don't
really know how those work, so this is what you get for now :)  It's pretty
neat, at least.

main.py

@@ -50,9 +50,11 @@ sshuttle --firewall <port> <subnets...>
 sshuttle --server
 --
 l,listen=  transproxy to this ip address and port number [default=0]
-N,auto-nets automatically determine subnets to route
+H,auto-hosts scan for remote hostnames and update local /etc/hosts
+N,auto-nets  automatically determine subnets to route
 r,remote=  ssh hostname (and optional username) of remote sshuttle server
 v,verbose  increase debug message verbosity
+seed-hosts= with -H, use these hostnames for initial scan (comma-separated)
 noserver   don't use a separate server process (mostly for debugging)
 server     [internal use only]
 firewall   [internal use only]
@@ -80,9 +82,18 @@ try:
         remotename = opt.remote
         if remotename == '' or remotename == '-':
             remotename = None
+        if opt.seed_hosts and not opt.auto_hosts:
+            o.fatal('--seed-hosts only works if you also use -H')
+        if opt.seed_hosts:
+            sh = re.split(r'[\s,]+', (opt.seed_hosts or "").strip())
+        elif opt.auto_hosts:
+            sh = []
+        else:
+            sh = None
         sys.exit(client.main(parse_ipport(opt.listen or '0.0.0.0:0'),
                              not opt.noserver,
                              remotename,
+                             sh,
                              opt.auto_nets,
                              parse_subnets(extra)))
 except Fatal, e: