Implements most of #17, except that we would like to restore OA_VARY
with the log read...

This should be backwards and forwards compatible, because the default
in both cases is 0, and varnish-cache will not use the bitfield
if it is 0.

 Conflicts:
	src/fellow_cache.c

 because fellow_disk_obj_compat() in master but not in 7.3