net: Add GetSysfd to conn type of net for getting sysfd for further use

[sandyskies]

fix #25729
SetDeadline have a poor performance in the benchmark, maybe because of the timmer.
So I add a function to get the sysfd of the socket, that I can set the specify socket options. I use the following code to set timeout in the socket:
fd = conn.(*net.UDPConn).GetSysfd()
var tv syscall.Timeval
tv.Sec = 1
tv.Usec = 0
err := syscall.SetsockoptTimeval(fd, syscall.SOL_SOCKET, syscall.SO_RCVTIMEO, &tv)

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Message from Ian Lance Taylor:

Patch Set 1: Code-Review-2

Thanks, but we are not going to make this change. It will fail badly if you hold onto the file descriptor but drop the Conn. It will not respect the locking protocol used by internal/poll. This implementation won't compile on Windows.

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Message from Ian Lance Taylor:

Patch Set 1:

I should add that you can already set the socket receive timeout via the SyscallConn method; call the Control method of the returned RawConn value.

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