1. Add option for socket buffer size for incoming messages on the tx side.
2. Check for socket buffer overflow and show warnings.
You can set socket buffer size system-wide via net.core.rmem_default or
via -R option in wfb_tx.
Now it is possible to use backup link via LTE modem or connect onboard computer without external mavlink-router.
To connect as backup link in QGC use tcp connection to port 5760 and check "high latency link" checkbox.
