Minor code refactoring of GC_register/move/unregister_disappearing_link

Minor code refactoring of GC_register/move/unregister_disappearing_link

* finalize.c (dl_set_next): Cast result to void.
* finalize.c (GC_general_register_disappearing_link,
GC_unregister_disappearing_link_inner,
GC_move_disappearing_link_inner, DELETE_DL_HASHTBL_ENTRY): Use
explicit comparison to NULL instead of "!" operator.
* finalize.c (FREE_DL_ENTRY): New macro.
* finalize.c (GC_unregister_disappearing_link_inner): Use
FREE_DL_ENTRY.
* finalize.c (GC_move_disappearing_link_inner): Add comment; use NULL
instead of 0 for pointers.
* finalize.c (GC_dump_finalization, ITERATE_DL_HASHTBL_BEGIN,
GC_finalize, GC_enqueue_all_finalizers): Remove redundant parenthesis
in expression for dl_size/fo_size calculation.
* finalize.c (DELETE_DL_HASHTBL_ENTRY): Add outermost '{', '}' braces.
* finalize.c (GC_make_disappearing_links_disappear_inner,
GC_remove_dangling_disappearing_links_inner): Move outermost '{', '}'
braces to ITERATE_DL_HASHTBL_BEGIN/END, respectively; add trailing ';'
for DELETE_DL_HASHTBL_ENTRY call.