Add ListenBacklog directive to control the backlog passed to listen().

Also change the default to 511 for platforms that use an 8-bit datatype
to store it.

A slightly different implementation than suggested by Taso Devetzis
<devetzis@snet.net>, who submitted the PR.

PR: 240
Reviewed by:	Dean Gaudet, Jim Jagielski
Submitted by:
Obtained from:


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@78429 13f79535-47bb-0310-9956-ffa450edef68