Use time-limited polling loops to verify parent and child startup and child

Use time-limited polling loops to verify parent and child startup and child
shutdown, instead of relying on unordered events.  Ignore anything that
appears on varnishd's std{out,err} except for an unexpected "child died",
which we handle by dying instead of emitting an event.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1878 d4fa192b-c00b-0410-8231-f00ffab90ce4