Add VCC support for backend probe specifications.

Add VCC support for backend probe specifications.

It is possible to specify just the URL to be probed, or to specify the
entire HTTP request:

	backend b0 {
		.host = "127.0.0.1";
		.probe = {
			.url = "/test.jpg";
			.timeout = 224 ms;
		}
	}

	backend b1 {
		.host = "127.0.0.1";
		.probe = {
			# NB: \r\n automatically inserted after each string!
			.request =
			    "GET / HTTP/1.1"
			    "Host: www.foo.bar"
			    "Connection: close";
			.timeout = 0.3 s;
		}
	}

If the full request has not been completed within the timeout, the
backend will be set to down.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2902 d4fa192b-c00b-0410-8231-f00ffab90ce4