Refactor capptr_domesticate SFINAE to make more statically safe.

This refactoring was provided by David.  Previously if a backend
provided a capptr_domesticate function with the wrong type it would be
silently ignored.  This change requires backends to explicitly opt in
to domestication via a new Backend::Option and ensures the compiler
will loudly complain if there is a mismatch.

src/test/func/domestication/domestication.cc

@@ -40,6 +40,7 @@ namespace snmalloc
     {
       Flags opts = {};
       opts.QueueHeadsAreTame = false;
+      opts.HasDomesticate = true;
       return opts;
     }
     ();