Make room in the shared memory segment for a panic string

Make room in the shared memory segment for a panic string
buffer.

Replace the default libvarnish assert handler with a child specific
function.

This function which fills the static panic string and copy the
result to the shared memory panicstring.

In the manager process, report the content of the panic string
when the child dies.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2971 d4fa192b-c00b-0410-8231-f00ffab90ce4