esp8266/modules/webrepl: Enforce only one concurrent WebREPL connection.

Concurrent WebREPL connections were never supported, now actually check
for this.

esp8266/modules/webrepl.py

@@ -31,6 +31,10 @@ def setup_conn(port, accept_handler):
 def accept_conn(listen_sock):
     global client_s
     cl, remote_addr = listen_sock.accept()
+    if uos.dupterm():
+        print("\nConcurrent WebREPL connection from", remote_addr, "rejected")
+        cl.close()
+        return
     print("\nWebREPL connection from:", remote_addr)
     client_s = cl
     websocket_helper.server_handshake(cl)