tcpin blocks when non-MAVLink client connects

Description:
When using --out=tcpin:0.0.0.0:14551, if a non-MAVLink client (e.g. telnet, browser) connects, MAVProxy accepts the connection but never receives valid MAVLink data. After that, real GCS clients cannot connect anymore, so MAVProxy becomes unusable.

Expected behavior:

If no MAVLink heartbeat is received within a short timeout (e.g. 5–10s), the connection should be closed.

Or allow multiple clients on tcpin instead of blocking after the first connection.

Steps to reproduce:

Run mavproxy.py --out=tcpin:0.0.0.0:14551

Connect with telnet localhost 14551

Real GCS cannot connect anymore.

Start with the mavproxy.py entry point and reproduce the issue using --out=tcpin:0.0.0.0:14551, then connect with telnet while observing how the tcpin connection is handled. Done means a non-MAVLink connection no longer prevents a real GCS client from connecting, either by closing it after the stated timeout or by allowing multiple clients.

Written by the indexing model from the issue text.