Add code to VCC to output an identifier for each simple backend which

Add code to VCC to output an identifier for each simple backend which
varnishd can use to decide if a backend state can be shared between
multiple VCL programs.

Originally the idea was to calculate a strong hash over the tokens
which define the backend.

Considering the relatively short lengths of backend declarations
and the infrequency of comparisons, we have opted for an extremely
weak hash instead:  We simply output the space separted tokens as
a string.

The net change in code for simple backends is marginal, but for
complex backends this will be a lot simpler to implement.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2007 d4fa192b-c00b-0410-8231-f00ffab90ce4