The log_labels logic is based on setup_traffic_protection in picotls.c.
Enables QUIC decryption with Wireshark v2.9.0rc0-1850-g2fd42045f5.

The file handle will be leaked on exit, since this is a debug feature I
am not too worried about that.

Implement connection migration

demo: do not crash if server context could not be created

Add logging of retry and negotiation packets in server log.

Remove not working IPv4 DONT FRAG attempt on Linux

Fixes scan-build warnings