Add two run-time parameters, "user" and "group", which specify an unprivileged

Add two run-time parameters, "user" and "group", which specify an unprivileged
user and group to which the child process will switch immediately after fork()
returns, before it starts accepting connections.  The default values are
"nobody" and "nogroup" (they should probably be tweakable at compile time...)

Note that this does not provide full privilege separation, as there are still
channels between the parent and child processes which need to be monitored,
but it is an improvement on the previous situation.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1482 d4fa192b-c00b-0410-8231-f00ffab90ce4