Allow to connect on a CLI during a soft-stop or a reload

[Willy Tarreau]

This patch series allows you to connect to a leaving process from the
master CLI, which might be convenient for debugging purpose :-)

Now merged, thank you. However when I try it I get an error message
indicating that it cannot find the socket, but I suppose this is
normal at this stage :

master> show proc
# <PID> <type> <relative PID> <reloads>
3414 master 0 -1
3441 worker 1 0
3437 worker 1 1

master> @-1 show info
Can't find the target CLI!

Then it quits.

Thanks,
Willy