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Make sure the terminal returned is live.
(create_terminal): Use allocate_terminal.
(mark_terminals): Move to alloc.c.
(delete_terminal): Use terminal->name as liveness status.
NULL out fields after freeing their contents.
Don't deallocate the object.
(Fframe_terminal): Use FRAME_TERMINAL.  Return the terminal object
rather than an int.
(Fterminal_live_p): Accept non-integer arguments.
(Fterminal_list): Return terminal objects rather than an ints.