Move the "simple" director into cache_backend.c, since it is what
is really dealing with backends now that directors are objectified.

Make a lot of stuff static as a result.

Collect both the init and fini VRT functins for directors in
cache_backend_cfg.c



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4413 d4fa192b-c00b-0410-8231-f00ffab90ce4