[udash] Run shell command on server through web ui

[at15]

This is similar to #41 except it's using os/exec and proxy stdin/out/err

Ref

• https://github.com/gravitational/console-demo use xterm and native websocket with pty
• https://github.com/gorilla/websocket/tree/master/examples/command for gorilla websocket
• https://github.com/yudai/gotty ts, xterm (also mentioned hterm ...) and gorilla websocket

[at15]

the key findings are backend implements varies based on how front end interact https://github.com/dyweb/blog/issues/39

• if you send a single command as a message from client, you need to add \n and can buffer output
• if you use xterm, you can't add \n and should NOT buffer output, xterm relies pty's echo back to show what you type and not buffering output allows it to works like a real terminal

[at15]

copied from PR ...