MINOR: sock: add a check against cross worker<->master socket activities

MINOR: sock: add a check against cross worker<->master socket activities

Given that the previous issues caused spurious worker socket wakeups in
the master for inherited FDs that couldn't be closed, let's add a strict
test in the I/O callback to make sure that an accept() event is always
caught by the appropriate type of process (master for master listeners,
worker for worker listeners).