Bind listener ports early and start accepting connections later

It may happen that auth or rdbms client tcp connections bind a local
socket to a port number required by a configered listener. The ejabberd
applications fails to start up and needs to be restarted.

In plain C you would bind(2) the listener port and listen(2) later on.
gen_tcp:listen/2 does not allow to separate these two steps though, so
another way is not to accept connections while start up. OTOH, the
kernel will syn/ack incoming connections and receive data, leaving them
in a buffer for the ejabberd to read from. If this is unwanted, a load
balancer would need to receive data from the ejabberd server before
adding the node to its pool.

This patch binds tcp ports while initializing the ejabberd_listener
process, storing ListenSockets in an ets table. start_listeners/0 will
reuse these ports later on.

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