Improve handling of dead hosts and closed connections

After each broctl command finishes, remove closed connections from the
list of connections and remove all hosts in the dead hosts list (then
broctl will automatically try to reestablish those connections when the
next broctl command runs rather than the user having to exit and rerun
broctl).  This change has no effect when broctl runs only a single command.

Also, fixed a problem where redundant "connection broke" or "not alive"
messages would appear by only reporting the first time (for each run
of a single command) such an event occurs.