Use PMTUD to set the MTU to the VPN interface

- Add `VPN_PMTUD` option enabled by default
- One can revert to use `VPN_PMTUD=off` to disable the new PMTUD mechanism

internal/pmtud/errors.go

@@ -1,10 +1,24 @@
 package pmtud
 
 import (
+	"context"
 	"errors"
+	"fmt"
+	"net"
+	"time"
 )
 
 var (
 	ErrICMPDestinationUnreachable = errors.New("ICMP destination unreachable")
 	ErrICMPBodyUnsupported        = errors.New("ICMP body type is not supported")
 )
+
+func wrapConnErr(err error, timedCtx context.Context, pingTimeout time.Duration) error { //nolint:revive
+	switch {
+	case errors.Is(timedCtx.Err(), context.DeadlineExceeded):
+		err = fmt.Errorf("%w (timed out after %s)", net.ErrClosed, pingTimeout)
+	case timedCtx.Err() != nil:
+		err = timedCtx.Err()
+	}
+	return err
+}