Change HASH_UNUSED_ENTRY_KEY from Qunbound to NULL float

Change HASH_UNUSED_ENTRY_KEY from Qunbound to NULL float

This removes hacks from code that had to be careful not to use
Qunbound as a hash table key, at the cost of a minor hack in
the GC marker.

* src/lisp.h (INVALID_LISP_VALUE, HASH_UNUSED_ENTRY_KEY):
Define as a null-pointer float.
* src/alloc.c (process_mark_stack): Add hack to ignore that value.
* src/pdumper.c (dump_object_needs_dumping_p)
(pdumper_init_symbol_unbound, pdumper_load):
* src/print.c (PRINT_CIRCLE_CANDIDATE_P): Remove hacks for Qunbound.