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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