Fix subtle errors with let-binding of localized variables

Fix subtle errors with let-binding of localized variables

* src/eval.c (do_specbind): Don't require a "symbol" that is
actually a cons cell, in order to call set-default, as there are
no longer such bindings.  This makes do_specbind work like the
pre-concurrency implementation in specbind for bindings of
forwarded symbols.  Use specpdl_kind to access the type of the
binding.
(specpdl_kind): New function.