Implement runtime part of VCL controlled hashing.

Implement runtime part of VCL controlled hashing.

The vcl_hash() is now used to control which fields go
into the hash algorithm, and the default is stil,
as previously, the URL + Host: header.

But now it is controlled by the vcl code, with the
default vcl_hash() being:

	sub vcl_hash {
		req.hash += req.url;
		req.hash += req.http.host;
		hash;
	}

Once I get a bit further, this will be changed to

	sub vcl_hash {
		req.hash += req.url;
		if (req.http.host) {
			req.hash += req.http.host;
		} else {
			req.hash += server.ip;
		}
		hash;
	}

So that we correctly hash HTTP requests without Host:
headers, that go to a machine with multiple IP numbers.

If you want to add fields to the hash, just write
a vcl_hash that does not end in "hash;":

	sub vcl_hash {
		req.hash += req.http.cookie;
	}

If you want to override the default vcl_hash, just
say so:

	sub vcl_hash {
		req.hash += req.url;
		hash;	// do not continue into default vcl_hash
	}

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1398 d4fa192b-c00b-0410-8231-f00ffab90ce4