send transit abilities to far end too, for the future

This will allow a future peer to figure out what transit modes we can
and cannot do, and thus avoid spinning up expensive modes that we won't
be able to use (e.g. WebRTC).

src/wormhole/cli/cmd_receive.py

@@ -149,8 +149,11 @@ class TwistedReceiver:
         tr.set_transit_key(transit_key)
 
         tr.add_connection_hints(sender_transit.get("hints-v1", []))
+        receiver_abilities = tr.get_connection_abilities()
         receiver_hints = yield tr.get_connection_hints()
-        receiver_transit = {"hints-v1": receiver_hints}
+        receiver_transit = {"abilities-v1": receiver_abilities,
+                            "hints-v1": receiver_hints,
+                            }
         self._send_data({u"transit": receiver_transit}, w)
         # TODO: send more hints as the TransitReceiver produces them
 

src/wormhole/cli/cmd_send.py

@@ -106,8 +106,12 @@ class Sender:
             self._transit_sender = ts
 
             # for now, send this before the main offer
-            hints = yield ts.get_connection_hints()
-            self._send_data({u"transit": {"hints-v1": hints}}, w)
+            sender_abilities = ts.get_connection_abilities()
+            sender_hints = yield ts.get_connection_hints()
+            sender_transit = {"abilities-v1": sender_abilities,
+                              "hints-v1": sender_hints,
+                              }
+            self._send_data({u"transit": sender_transit}, w)
 
             # TODO: move this down below w.get()
             transit_key = w.derive_key(APPID+"/transit-key",

src/wormhole/transit.py

@@ -602,6 +602,11 @@ class Common:
         ep = endpoints.serverFromString(reactor, "tcp:%d" % portnum)
         return direct_hints, ep
 
+    def get_connection_abilities(self):
+        return [{u"type": u"direct-tcp-v1"},
+                {u"type": u"relay-v1"},
+                ]
+
     @inlineCallbacks
     def get_connection_hints(self):
         hints = []