Teach the VCL compiler about default functions, so that users will

Teach the VCL compiler about default functions, so that users will
not have to copy&paste the default methods if they have no special
requirements for a particular method.

No such facility exists for backends, so a backend description is
now the minumum VCL program.

When we initialize the VCL compiler we hand it a piece of source code
with the "default code", this must include definitions of all methods
named with a "default_" prefix (ie: "default_vcl_recv" etc).

During compilation we always compile this piece of source code in (after
the user supplied VCL source).

If the user did not provide a particular method, the default method is
used instead.  The user can also call the default method directly,
for instance by:

	sub vcl_recv {
		if (req.http.Expect) {
			error;
		}
		call default_vcl_recv;
	}

Later on, this could be expanded to allow other subroutines to be
included in the default VCL for the users calling convenience.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@449 d4fa192b-c00b-0410-8231-f00ffab90ce4