cmdserver: add option to not exit from message loop on SIGINT
Sending SIGINT to server is the only way to interrupt a command running in
command-server process. SIGINT will be caught at dispatch.dispatch() if
we're lucky. Otherwise it will terminate the serer process. This is
fundamentally unreliable as signals are delivered asynchronously.
"cmdserver.shutdown-on-interrupt=False" mitigate the issue by making the
server basically block SIGINT.