Honour the packet size limit in bare-connection protocol.

When I set up the simplified version of just the ssh-connection
protocol we use for connection sharing, I carefully documented at the
top of sshshare.c that packets in that protocol variant are limited to
just over 0x4000 bytes. And did I remember to actually honour that, by
restricting the sizes of outgoing packets when actually speaking the
bare connection protocol? I did not.

Well, better late than never. Here I introduce a packet size limit
that can be imposed by the BPP, and arrange for sshconnection.c to
take the min of that and any given channel's max packet size as sent
by the remote end. All BPPs except ssh2bpp-bare set it to the no-op
value of the largest possible uint32_t.

sshverstring.c

@@ -51,6 +51,7 @@ static const struct BinaryPacketProtocolVtable ssh_verstring_vtable = {
     ssh_verstring_handle_output,
     ssh_verstring_new_pktout,
     ssh_verstring_queue_disconnect,
+    0xFFFFFFFF, /* no special packet size limit for this bpp */
 };
 
 static void ssh_detect_bugs(struct ssh_verstring_state *s);