Add libxml2 to set of libraries we package with the tarball.

This is used by the cream_gahp.
Make listen() backlog configurable with SOCKET_LISTEN_BACKLOG. #4393

New configuration parameter SOCKET_LISTEN_BACKLOG can be used to set the
listen() backlog used in daemons. The default of 500 maintains the old
behavior.
Docs for new config param SOCKET_LISTEN_BACKLOG. #4393