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.

server.py

@@ -1,6 +1,6 @@
-import re, struct, socket, select, subprocess
+import re, struct, socket, select, subprocess, traceback
 if not globals().get('skip_imports'):
-    import ssnet, helpers
+    import ssnet, helpers, hostwatch
     from ssnet import SockWrapper, Handler, Proxy, Mux, MuxWrapper
     from helpers import *
 
@@ -65,7 +65,38 @@ def list_routes():
     for (ip,width) in _list_routes():
         if not ip.startswith('0.') and not ip.startswith('127.'):
             yield (ip,width)
-        
+
+
+def _exc_dump():
+    exc_info = sys.exc_info()
+    return ''.join(traceback.format_exception(*exc_info))
+
+
+def start_hostwatch(seed_hosts):
+    s1,s2 = socket.socketpair()
+    pid = os.fork()
+    if not pid:
+        # child
+        rv = 99
+        try:
+            s2.close()
+            os.dup2(s1.fileno(), 1)
+            os.dup2(s1.fileno(), 0)
+            s1.close()
+            rv = hostwatch.hw_main(seed_hosts) or 0
+        except Exception, e:
+            log('%s\n' % _exc_dump())
+            rv = 98
+        finally:
+            os._exit(rv)
+    s1.close()
+    return pid,s2
+
+
+class Hostwatch:
+    def __init__(self):
+        self.pid = 0
+        self.sock = None
 
 
 def main():
@@ -93,15 +124,36 @@ def main():
                        for r in routes)
     mux.send(0, ssnet.CMD_ROUTES, routepkt)
 
+    hw = Hostwatch()
+
+    def hostwatch_ready():
+        assert(hw.pid)
+        content = hw.sock.recv(4096)
+        if content:
+            mux.send(0, ssnet.CMD_HOST_LIST, content)
+        else:
+            raise Fatal('hostwatch process died')
+
+    def got_host_req(data):
+        if not hw.pid:
+            (hw.pid,hw.sock) = start_hostwatch(data.strip().split())
+            handlers.append(Handler(socks = [hw.sock],
+                                    callback = hostwatch_ready))
+    mux.got_host_req = got_host_req
+
     def new_channel(channel, data):
         (dstip,dstport) = data.split(',', 1)
         dstport = int(dstport)
         outwrap = ssnet.connect_dst(dstip,dstport)
         handlers.append(Proxy(MuxWrapper(mux, channel), outwrap))
-    
     mux.new_channel = new_channel
 
     while mux.ok:
+        if hw.pid:
+            (rpid, rv) = os.waitpid(hw.pid, os.WNOHANG)
+            if rpid:
+                raise Fatal('hostwatch exited unexpectedly: code 0x%04x\n' % rv)
+        
         r = set()
         w = set()
         x = set()