introduce the target stack

there's a huge hack in multi_target_compatible

2014-07-29  Tom Tromey  <tromey@redhat.com>

	* target-delegates.c: Rebuild.
	* make-target-delegates (write_debugmethod): Update for change to
	debug_target.
	* progspace.c (add_program_space): Acquire a reference to the
	target stack.
	(release_program_space): Release reference to the program space.
	(set_current_program_space): Set the target stack.
	* progspace.h (struct program_space) <target_stack>: New field.
	* target.c (target_ops_ptr): New typedef.
	(struct target_stack): New.
	(target_stack): Change type.
	(current_target): Update comment.
	(update_current_target): Update.
	(multi_target_compatible, ensure_multi_target_ok): New functions.
	(push_target): Update.  Add multi-target check.
	(unpush_target, pop_all_targets_above, target_is_pushed)
	(target_info, target_require_runnable): Update.
	(target_stack_set): New global.
	(currently_multi_target, target_stack_incref, target_stack_decref)
	(target_stack_set_current, new_target_stack): New functions.
	(maintenance_print_target_stack): Update.
	(initialize_targets): Initialize target_stack_set, target_stack.
	Update.
	(target_stack_id): New function.
	(next_available_target_id): New global.
	(print_one_target_stack): New function.
	(maintenance_print_target_stack): Move earlier.  Rewrite.
	(debug_target): Remove.
	(debug_to_open, setup_target_debug): Update.
	* target.h (target_stack_incref, target_stack_decref)
	(target_stack_set_current, new_target_stack, target_stack_id):
	Declare.
        * testsuite/gdb.base/auto-connect-native-target.exp: Update.