BULK improvements and press BACK to exit

[Knifa]

• BULK setting removed: both BULK and RNDIS listen at the same time. You can use both freely and swap between them without restarting the shim.
• BULK connection loss timeout: BULK will now consider the connection lost if no data is received for 3 seconds.
• Press BACK to exit: you can now press BACK to exit the shim at any time.

Implementation wise, everything is now async/poll-based.