Fixed bug in actor spawn closures.

The closure in the block given to the actor_prim.spawn function could be
changed by the caller before the callee had a chance to extract its
values.

Fixed by resolving all pointers in the closure before the thread starts,
and storing them at a new glmalloced address. Changes to the pointers by
the caller are then no longer reflected in the closure.
Raise exception on post to dead actor.

If an actor is dead, then post (or ! in the high-level API) will raise
an exception.