Make listen backlog customizable

Excerpt from a mail we got on the list:

On 02/19/2015 03:07 PM, Stephan Leemburg wrote:

I am working for the NPO, the Dutch Public Broadcasting agency.

We do a lot of icecast streaming. We run at least 20 icecast server instances on our media streaming cluster. [...]

We ran into an issue that clients which where connecting to our streams seemed to be 'hanging' on the connection setup frequently. The client 'thinks' it is connected, but no data.

People suggested that it probably had to do with the poll() call. So, I looked into that.

I found that the issue was actually caused by the very low listen backlog (5). On our clusters, we typically set this to 8192. Yes it is high, but we do a lot of streaming and host very high volume websites.

Attached are the submitted patches for 2.4, 2.5 and 2.3.3