Suspending and killing mosh-client may not terminate sessions properly

Ideally, when you suspend the Mosh client with ^^ ^Z and kill it with kill %1, mosh-client would properly terminate, including sending a shutdown packet to the server to close the shell session. But on at least some systems (including my Mac and RPi), the kill causes mosh-client to exit immediately with tcsetattr: Input/output error. We should handle that error a little more gracefully.

Related: Currently our network code sends shutdown messages by setting a shutdown_in_progress state variable which tells the code to mark the next sent packet as a shutdown packet, and that packet may not get sent immediately. It might be good to have a synchronous method in the network code that immediately generates and sends a single shutdown packet, for situations where we don't want to wait for the full shutdown sequence.

Start by reproducing the suspend-and-kill sequence described for mosh-client and inspect the client termination path where tcsetattr reports an input/output error. Then examine the network shutdown handling and its shutdown_in_progress state. Done means the client exits gracefully and sends a shutdown packet that closes the server-side shell session.