We didn't create/destroy directors and backends in a consistent order,
and in some case we even destroyed directors more than once.

Always destroy in opposite order of creation (which follows VCL
source order).

Turn the bottom element of the array into (only) an indication
of which backend/director is the default.

Fixes:	#722



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4967 d4fa192b-c00b-0410-8231-f00ffab90ce4