This commit implements the option to disable PTMU discovery for QUIC
connections.
QUIC finds the PMTU during startup by increasing Ping packet frames
until Ping responses are not received anymore, and it seems to stick
with that PMTU forever.
This is no problem if the PTMU doesn't change over time, but if it does
it may case packet drops.
We add this hidden flag for debugging purposes in such situations as a
quick way to validate if problems that are being seen can be solved by
reducing the packet size to the edge.
Note however, that this option may impact UDP proxying since we expect
being able to send UDP packets of 1280 bytes over QUIC.
So, this option should not be used when tunnel is being used for UDP
proxying.
0c8bc56930