[MEDIUM] listeners: put listeners in queue upon resource shortage

When an accept() fails because of a connection limit or a memory shortage,
we now disable it and queue it so that it's dequeued only when a connection
is released. This has improved the behaviour of the process near the fd limit
as now a listener with a no connection (eg: stats) will not loop forever
trying to get its connection accepted.

The solution is still not 100% perfect, as we'd like to have this used when
proxy limits are reached (use a per-proxy list) and for safety, we'd need
to have dedicated tasks to periodically re-enable them (eg: to overcome
temporary system-wide resource limitations when no connection is released).

include/types/global.h

@@ -123,6 +123,7 @@ extern const struct linger nolinger;
 extern int stopping;	/* non zero means stopping in progress */
 extern char hostname[MAX_HOSTNAME_LEN];
 extern char localpeer[MAX_HOSTNAME_LEN];
+extern struct list global_listener_queue; /* list of the temporarily limited listeners */
 
 #endif /* _TYPES_GLOBAL_H */