Function to mark objects and remove killed buffers at once.

Function to mark objects and remove killed buffers at once.
* alloc.c (discard_killed_buffers): Rename to ...
(mark_discard_killed buffers) ... new name.  Add marking
of remaining objects.  Fix comment.  Adjust users.
(mark_object): Do not touch frame buffer lists here.
* frame.c (delete_frame): Reset frame buffer lists here.