Fixes for --invoke for v0.9

[tonistiigi]

I think it is worth doing some small updates to --invoke so we can get better feedback when we release this as experimental in v0.9.

• On initial launch of the debug container, a message should be printed "Launching interactive container. Press Ctrl-a-c to switch to monitor console".
• There should be "help" command that lists the currently available commands. Typing unknown command should also show help or show "Run help to see all supported commands".

Nice to have:

• Output warnings should be suppressed then running --invoke. "WARNING: No output specified " is not relevant.
• After issuing "rollback" command would be nice to show message "Interactive container was restarted. Press Ctrl-a-c to switch to the new container".
• On monitor mode, if there is no active container and Ctrl-a-c is pressed, a message could show "No running interactive containers. You can start one by issuing rollback command".

@ktock (or others) LMK if you would like to take some of these; otherwise, I can look into them myself as well.

[ktock]

Thanks for the suggestion. I'll take it.