- remove the obj.backend variable, which is not connected to anything.
 - define a req.backend variable and implement l/r functions for it
 - complete / correct support for setting / comparing backend values

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@778 d4fa192b-c00b-0410-8231-f00ffab90ce4