Add assertions to finalize and threads support for MANUAL_VDB needs

Add assertions to finalize and threads support for MANUAL_VDB needs
(code refactoring)

* finalize.c (GC_unregister_disappearing_link_inner,
GC_enqueue_all_finalizers): Remove redundant comment about held lock.
* finalize.c (GC_unregister_disappearing_link_inner,
GC_move_disappearing_link_inner, ITERATE_DL_HASHTBL_BEGIN,
GC_finalize, GC_enqueue_all_finalizers): Add assertion that the
allocation lock is held.
* pthread_support.c (first_thread): Refine comment.
* pthread_support.c (GC_new_thread): Add assertion that when the first
thread is added then GC_threads[hv] is null (thus "next" field is set
to null).
* win32_threads.c (GC_new_thread): Likewise.
* pthread_support.c (GC_delete_thread, GC_delete_gc_thread): Add
assertion that "next" field of the first thread is not modified.
* win32_threads.c (GC_delete_gc_thread_no_free, GC_delete_thread):
Likewise.
* win32_threads.c (GC_delete_thread): Use EXPECT() when checking that
p is not the first thread.
* win32_threads.c [GC_PTHREADS] (GC_pthread_start_inner): Add assertion
that the current thread is not the first one (thus "status" field of
the first thread is not modified).