(struct backtrace): Add debug_on_exit member.

(Fgarbage_collect): Clear out buffer undo_list markers after gc_sweep.
Identify those markers as Lisp_Misc_Free objects.  Clear car and cdr of
the removed cons cells.
(mark_object): Undo previous change - disallow Lisp_Misc_Free objects.
(gc_sweep): Clear cons_blocks before sweeping strings, so we don't have
any cons cells pointing to unallocated stings.
Do not lisp_free any marker blocks, as there may still be pointers
to them from buffer undo lists at this stage of GC.

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