Add support for piping into wsl-sudo.py

[kajic]

Make the following work:

echo "foo" | ./wsl-sudo.py cat

Without these changes, cat would hang indefinitely on the pipe.

Fix the issue by:

1. Prevent the server process from inheriting the client's stdin, by explicitly setting its stdin to subprocess.DEVNULL. The primary reason for this is to prevent the server process from messing up the client's stdin pipe. It's unclear if there is an issue with wsl, but letting the server inherit the pipe seems to consume it, leaving nothing for the client to read.
   Besides, the server receives all of its input from the client-server socket, so it doesn't even need the client's stdin.
2. Send an empty string CMD_STDIN command from the client to the server when a empty chunk is read from stdin. The client already uses 'empty string' to detect a closed pipe, and now, by also transmitting an empty string to the server, the server can do the same.
3. Receive the empty string CMD_STDIN in the server's transfer loop, closing the server's child process stdin in response.