fix(redirect): add IPv6 TPROXY support for TCP redirect listener

Add missing IPV6_TRANSPARENT socket option to the TCP redirect listener
Control callback, matching what the UDP redirect listener already sets.
Without this, the kernel silently drops IPv6 TPROXY-redirected connections.

Also clean up handler_linux.go getOriginalDstAddr:
- Remove encoding/binary import and heap allocation for port conversion
- Replace with zero-allocation ntohs (native-to-network byte order swap)
- Document the struct layout hack where GetsockoptIPv6Mreq and
  GetsockoptIPv6MTUInfo are used to read sockaddr_in/sockaddr_in6
  from SO_ORIGINAL_DST (kernel returns smaller structs than the
  getter functions expect, but only the first N bytes are populated)

Fixes go-gost/gost#126

listener/redirect/tcp/listener_linux.go

@@ -11,5 +11,8 @@ func (l *redirectListener) control(network, address string, c syscall.RawConn) e
 		if err := unix.SetsockoptInt(int(fd), unix.SOL_IP, unix.IP_TRANSPARENT, 1); err != nil {
 			l.logger.Errorf("SetsockoptInt(SOL_IP, IP_TRANSPARENT, 1): %v", err)
 		}
+		if err := unix.SetsockoptInt(int(fd), unix.SOL_IPV6, unix.IPV6_TRANSPARENT, 1); err != nil {
+			l.logger.Errorf("SetsockoptInt(SOL_IPV6, IPV6_TRANSPARENT, 1): %v", err)
+		}
 	})
 }