This patch introduces the concept of socket privilege levels. "UPDATE"

This patch introduces the concept of socket privilege levels.  "UPDATE"
and "FLUSHALL" commands are restricted to high-privilege sockets.  "FLUSH"
commands can be executed on any socket.  This is ideal for multi-user
installations where only certain users need write access to the RRD files.

Now, nearly all socket information is passed around the daemon in
listen_socket_t data structures.  In case there is other per-socket state
(i.e. if we add authentication) we can put it there.

Also, I created a new "open_listen_socket_network" and removed the network
setup from "open_listen_socket". -- kevin

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk@1547 a5681a0c-68f1-0310-ab6d-d61299d08faa