Fix SIGSEGV in GC_is_marked when gc_cleanup is used in leak-finding mode

Fix SIGSEGV in GC_is_marked when gc_cleanup is used in leak-finding mode

Issue #162 (bdwgc).

Now finalizers and disappearing links registration is a no-op in case
of the leak-finding mode.

*  finalize.c (GC_register_disappearing_link_inner): Do nothing (return
GC_UNIMPLEMENTED) if GC_find_leak.
* finalize.c (GC_register_finalizer_inner): Do nothing if GC_find_leak.
* include/gc.h (GC_find_leak): More verbose comment.
* include/gc.h (GC_debug_register_finalizer,
GC_general_register_disappearing_link): Document the case of
GC_find_leak.