Add 'GC_' prefix to dl_head and fo_head (code refactoring)

Add 'GC_' prefix to dl_head and fo_head (code refactoring)

* finalize.c (HASH2): Enclose argument in parentheses.
* finalize.c (dl_set_next, fo_set_next): Enclose the whole expression
in parentheses.
* finalize.c (dl_head, fo_head): Add GC_ prefix; use STATIC instead of
"static" (to make the symbols globally visible during debugging).
* finalize.c (log_dl_table_size, GC_finalize): Reformat the comment.
* finalize.c (GC_push_finalizer_structures,
GC_general_register_disappearing_link,
GC_unregister_disappearing_link, GC_move_disappearing_link,
GC_dump_finalization, GC_finalize): Rename dl_head to GC_dl_head.
* finalize.c (GC_push_finalizer_structures,
GC_register_finalizer_inner, GC_dump_finalization, GC_finalize,
GC_enqueue_all_finalizers); Rename fo_head to GC_fo_head.
* finalize.c (GC_general_register_disappearing_link,
GC_unregister_disappearing_link, GC_dump_finalization, GC_finalize):
Reformat code.