listen_for_commands: stop leaking the control socket to forked procs

All the exec'ed processes would get copies of the control socket
descriptors, because of the default UNIX semantics of fd inherit across
execv(). This was easily seen on my system where an 'lsof' revealed
that all my terminals and shells had a copy.

To fix this we add the SOCK_CLOEXEC flag whilst opening the listener
socket(), avoiding this problem. We also need to set CLOEXEC on the
socket returned by accept():

"accept() ... extracts the first connection request on the queue of
pending connections ... creates a new connected socket, and returns a
new file descriptor"

The new descriptor does not inherit CLOEXEC flag from the listener.

The "bar" descriptor already handles this by setting O_CLOEXEC in its
own open() call on the FIFO.