Initial values of special bindings on thread startup

[ehuelsmann]

We don't set any special variables upon thread startup. This may or may not (to be determined) be fine, however, libraries can't specify their own, nor do we allow users to specify values.

CCL uses an :INITIAL-BINDINGS argument for its thread creation function.

Note: How to handle threads which aren't started from Lisp, but instead just get 'associated' with Lisp? Those should probably create a set of default bindings. This situation occurs when a thread is created outside the new LispThread?() interface, but currentThread() is called as part of the code running on it.