cmd_receive: treat 'quit' or other non- yes/no answers as "no"

So "yes" or "Yes" or "y" or "Y" or anything else that starts with y/Y means
yes, and a bare newline means yes, but anything else means no.

src/wormhole/cli/cmd_receive.py

@@ -370,7 +370,7 @@ class Receiver:
         with self.args.timing.add("permission", waiting="user") as t:
             while True and not self.args.accept_file:
                 ok = six.moves.input("ok? (Y/n): ")
-                if not ok.lower().startswith("n"):
+                if ok.lower().startswith("y") or len(ok) == 0:
                     if os.path.exists(self.abs_destname):
                         self._remove_existing(self.abs_destname)
                     break