Problem: libzmq does not send ZMTP 3.1 sub/cancel commands

Solution: if all peers of a socket are >= 3.1 use sub/cancel commands
instead of the old 0/1 messages.
For backward compatibility, move the handling of 0/1 or sub/cancel
command strings to the encoders, so that the right thing can be done
depending on the protocol version.
Do not set the command flag until the encoder, so that we can handle
the inproc case (which skips the encoder).

src/v1_encoder.hpp

@@ -46,7 +46,7 @@ class v1_encoder_t ZMQ_FINAL : public encoder_base_t<v1_encoder_t>
     void size_ready ();
     void message_ready ();
 
-    unsigned char _tmpbuf[10];
+    unsigned char _tmpbuf[11];
 
     ZMQ_NON_COPYABLE_NOR_MOVABLE (v1_encoder_t)
 };