This commit introduces additional platform-dependent checking when the
kernel returns an error. Previously, the session is terminated when
PingFrame sends a discovery packet larger than the limit. With this
commit, the error is checked, and if it is "datagram too large", the
error is ignored.
Additionally,
1. This commit re-enables MTU discovery on Windows unless it
is disabled explicitly by user (Undo #3276),
2. Set IP_DONTFRAGMENT and IPV6_DONTFRAG with error checking on Windows,
and
3. Set IP_MTU_DISCOVERY to PMTUDISC_DO for both IPv4 and IPv6 on Linux
so that the kernel will return "message too long".
Fixes#3273#3327
The syscall setting the IP_DONTFRAGMENT socket option will fail if the
underlying connection is a IPv6 connection. Since Go doesn't allow us to
distinguish between IPv6 and dual-stack connections, we can only ignore the
error here.
