have to kill the child:  We cannot get the CLI pipes back in sync,
even if the child process was just preoccupied with something for 
much longer than cli_timeout.

Fixes: #744



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5116 d4fa192b-c00b-0410-8231-f00ffab90ce4