Handle different Unix domain socket (actually socketpair) semantics

Handle different Unix domain socket (actually socketpair) semantics
in BSD vs. Linux.  In BSD if one end of the socketpair goes away
select() returns the fd as readable and the read will fail with
ECONNRESET.  This doesn't appear to happen on Linux so if we notice
that the monitor process has died when I/O logging is enabled,
behave like the command has exited.  This means we log the wait
status of the monitor, not the command, but there is nothing else
we can do at that point.  This should only be an issue if SIGKILL
is sent to the monitor process.