* corelow.c (core_open): Assume there was no upper layer left

	* corelow.c (core_open): Assume there was no upper layer left
	behind from a previous inferior.
	* target.c (pop_all_targets): Rename to ...
	(pop_all_targets_above): ... this.  Add a target stratum
	parameter.  Use it instead of hardcoding the dummy_stratum.
	(pop_all_targets): New, defer to pop_all_targets_above.
	(target_preopen): Use pop_all_targets_above.
	* target.h (pop_all_targets_above): Declare.