If we get more HTTP headers than we have room for (default: 28) we

If we get more HTTP headers than we have room for (default: 28) we
used to ignore the rest.

This is not a bright solution if crucial HTTP headers like
"Content-Length" or "Transfer-Encoding" are last and get ignored.

In general, it is highly suspect to randomly ignore HTTP headers,
as opposed to deliberately ignoring them, either by having first
looked at them and found them uninteresting, or by having looked
for the headers we care about, and having not matched some others.

Change too many headers to firm error condition: 400 if from the
client, and 503 (like every other trouble) if from the backend.

Fixes #416

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3498 d4fa192b-c00b-0410-8231-f00ffab90ce4