remote: Shutdown if tls goes down while waiting for shutdown all-clear

When pacemaker-remoted wants to exit, it informs its connection host (if connected),
receives an ack, then waits for an all-clear message before exiting.

If the remote connection drops after sending the shutdown request and
receiving the ack, pacemaker-remoted will continue waiting for the
all-clear even though it can never arrive.

We now exit immediately with an exit status in that case.

fixes: T361