[BUG] stream_sock: cleanly disable the listener in case of resource shortage

[BUG] stream_sock: cleanly disable the listener in case of resource shortage

Jozsef R.Nagy reported a reliability issue on FreeBSD. Sometimes an error
would be emitted, reporting the inability to switch a socket to non-blocking
mode and the listener would definitely not accept anything. Cyril Bonté
narrowed this bug down to the call to EV_FD_CLR(l->fd, DIR_RD).

He was right because this call is wrong. It only disables input events on
the listening socket, without setting the listener to the LI_LISTEN state,
so any subsequent call to enable_listener() from maintain_proxies() is
ignored ! The correct fix consists in calling disable_listener() instead.

It is discutable whether we should keep such error path or just ignore the
event. The goal in earlier versions was to temporarily disable new activity
in order to let the system recover while releasing resources.