BUG/MEDIUM: listener: only enable a listening listener if needed

BUG/MEDIUM: listener: only enable a listening listener if needed

The test on listener->state == LI_LISTEN is not sufficient to decide
if we need to enable a listener. Indeed, there is a very special case
which is the inherited FD shared, which has to reflect the real socket
state even after the previous test, and as such needs to remain in
LI_LISTEN state. In this case we don't want a worker to start the
master's listener nor conversely. Let's add a specific test for this.