Don't allocate socket until we need it

Wew were trying to allocate an IPv6 socket even though we weren't using
IPv6, causing failures on systems without IPv6 support available.

This change means a number of methods on MultiListener, e.g. setsockopt,
should not be called until after the bind call.

Closes #68

sshuttle/client.py

@@ -95,16 +95,21 @@ def daemon_cleanup():
 class MultiListener:
 
     def __init__(self, type=socket.SOCK_STREAM, proto=0):
-        self.v6 = socket.socket(socket.AF_INET6, type, proto)
-        self.v4 = socket.socket(socket.AF_INET, type, proto)
+        self.type = type
+        self.proto = proto
+        self.v6 = None
+        self.v4 = None
+        self.bind_called = False
 
     def setsockopt(self, level, optname, value):
+        assert(self.bind_called)
         if self.v6:
             self.v6.setsockopt(level, optname, value)
         if self.v4:
             self.v4.setsockopt(level, optname, value)
 
     def add_handler(self, handlers, callback, method, mux):
+        assert(self.bind_called)
         socks = []
         if self.v6:
             socks.append(self.v6)
@@ -119,6 +124,7 @@ def add_handler(self, handlers, callback, method, mux):
         )
 
     def listen(self, backlog):
+        assert(self.bind_called)
         if self.v6:
             self.v6.listen(backlog)
         if self.v4:
@@ -133,16 +139,23 @@ def listen(self, backlog):
                     raise e
 
     def bind(self, address_v6, address_v4):
+        assert(not self.bind_called)
+        self.bind_called = True
         if address_v6 and self.v6:
+            self.v6 = socket.socket(socket.AF_INET6, self.type, self.proto)
+            self.v6.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             self.v6.bind(address_v6)
         else:
             self.v6 = None
         if address_v4 and self.v4:
+            self.v4 = socket.socket(socket.AF_INET, self.type, self.proto)
+            self.v4.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             self.v4.bind(address_v4)
         else:
             self.v4 = None
 
     def print_listening(self, what):
+        assert(self.bind_called)
         if self.v6:
             listenip = self.v6.getsockname()
             debug1('%s listening on %r.\n' % (what, listenip))
@@ -569,11 +582,9 @@ def main(listenip_v6, listenip_v4,
     for port in ports:
         debug2(' %d' % port)
         tcp_listener = MultiListener()
-        tcp_listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
 
         if required.udp:
             udp_listener = MultiListener(socket.SOCK_DGRAM)
-            udp_listener.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
         else:
             udp_listener = None