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.

ssnet.py

@@ -13,6 +13,8 @@ CMD_CLOSE = 0x4204
 CMD_EOF = 0x4205
 CMD_DATA = 0x4206
 CMD_ROUTES = 0x4207
+CMD_HOST_REQ = 0x4208
+CMD_HOST_LIST = 0x4209
 
 cmd_to_name = {
     CMD_EXIT: 'EXIT',
@@ -23,6 +25,8 @@ cmd_to_name = {
     CMD_EOF: 'EOF',
     CMD_DATA: 'DATA',
     CMD_ROUTES: 'ROUTES',
+    CMD_HOST_REQ: 'HOST_REQ',
+    CMD_HOST_LIST: 'HOST_LIST',
 }
     
 
@@ -223,6 +227,7 @@ class Mux(Handler):
         self.rsock = rsock
         self.wsock = wsock
         self.new_channel = self.got_routes = None
+        self.got_host_req = self.got_host_list = None
         self.channels = {}
         self.chani = 0
         self.want = 0
@@ -284,7 +289,17 @@ class Mux(Handler):
             if self.got_routes:
                 self.got_routes(data)
             else:
-                raise Exception('weird: got CMD_ROUTES without got_routes?')
+                raise Exception('got CMD_ROUTES without got_routes?')
+        elif cmd == CMD_HOST_REQ:
+            if self.got_host_req:
+                self.got_host_req(data)
+            else:
+                raise Exception('got CMD_HOST_REQ without got_host_req?')
+        elif cmd == CMD_HOST_LIST:
+            if self.got_host_list:
+                self.got_host_list(data)
+            else:
+                raise Exception('got CMD_HOST_LIST without got_host_list?')
         else:
             callback = self.channels[channel]
             callback(cmd, data)