Complete the separation of synthetic from received objects.

Complete the separation of synthetic from received objects.

	Any object, also errors, we receive from the backend will
	go to vcl_fetch(), and can be cached if desired.

	Any object we create, for instance if we cannot contact the
	backend, will go to vcl_error() and will not be cached.

Technical details:

	Implement VCL variable setting for booleans like obj.cacheable.

	Remove obj.valid, only valid objects go to vcl_fetch now.

	On fetch failure, drop the object and go to STP_ERROR with 503

	Update testcase b00015.vtc accordingly.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3044 d4fa192b-c00b-0410-8231-f00ffab90ce4