sendQueue: ignore "datagram too large" error (#3328)

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

send_queue.go

@@ -64,7 +64,13 @@ func (h *sendQueue) Run() error {
 			shouldClose = true
 		case p := <-h.queue:
 			if err := h.conn.Write(p.Data); err != nil {
-				return err
+				// This additional check enables:
+				// 1. Checking for "datagram too large" message from the kernel, as such,
+				// 2. Path MTU discovery,and
+				// 3. Eventual detection of loss PingFrame.
+				if !isMsgSizeErr(err) {
+					return err
+				}
 			}
 			p.Release()
 			select {