backend.

The new logic is:
	If (HEAD)	/* happens only on pass */
		do not fetch body.
	else if (Content-Length)
		fetch body according to length
	else if (chunked)
		fetch body as chunked
	else if (other transfer-encoding)
		fail
	else if (Connection: keep-alive)
		fetch no body, set Length = 0
	else if (Connection: close)
		fetch body until EOF
	else if (HTTP < 1.1)
		fetch body until EOF
	else
		fetch no body, set Length = 0

let me know if this breaks anything that should work.

Fixes #400



git-svn-id: http://www.varnish-cache.org/svn/trunk@3470 d4fa192b-c00b-0410-8231-f00ffab90ce4