r4356:
Don't halfclose the backend polling TCP connection after sending the
request, some backends gets confused by this.

Add a ".status" to backend polling, to configure the expected HTTP
status code for a good poll.

Fixes #584

r4357:
.expected_response is a better idea than .status

Tip o' the hat to: tfheen



git-svn-id: http://www.varnish-cache.org/svn/branches/2.0@4381 d4fa192b-c00b-0410-8231-f00ffab90ce4