Cut the thicket for setting a HTTP response's first line down to just
one function which does what you'd expect.

Start allowing internal VCL use of *resp.status > 999.  The idea
is that we ignore the higher bits but it still has to end up >=
100, so 10404 becomes 404 and 20099 is illegal.   (This change
is not yet complete it will panic if you use it yet.)

More http->failed stuff.