Replace code accessing list implementation details with API calls.

Replace code accessing list implementation details with API calls.

	* dll.c (clear_dlls): Replace accessing list implemention details
	with API function.
	* gdbthread.h (get_first_thread): Declare.
	* inferiors.c (for_each_inferior_with_data): New function.
	(get_first_thread): New function.
	(find_thread_ptid): Simplify.
	(get_first_inferior): New function.
	(clear_list): Delete.
	(one_inferior_p): New function.
	(clear_inferior_list): New function.
	(clear_inferiors): Update.
	* inferiors.h (for_each_inferior_with_data): Declare.
	(clear_inferior_list): Declare.
	(one_inferior_p): Declare.
	(get_first_inferior): Declare.
	* linux-low.c (linux_wait_for_event): Replace accessing list
	implemention details with API function.
	* server.c (target_running): Ditto.
	(accumulate_file_name_length): New function.
	(emit_dll_description): New function.
	(handle_qxfer_libraries): Replace accessing list implemention
	details with API function.
	(handle_qxfer_threads_worker): New function.
	(handle_qxfer_threads_proper): Replace accessing list implemention
	details with API function.
	(handle_query): Ditto.
	(visit_actioned_threads_callback_ftype): New typedef.
	(visit_actioned_threads_data): New struct.
	(visit_actioned_threads): Rewrite to be find_inferior callback.
	(resume): Call find_inferior.
	(handle_status): Replace accessing list implemention
	details with API function.
	(process_serial_event): Replace accessing list implemention details
	with API function.
	* target.c (set_desired_inferior): Replace accessing list implemention
	details with API function.
	* tracepoint.c (same_process_p): New function.
	(gdb_agent_about_to_close): Replace accessing list implemention
	details with API function.
	* win32-low.c (child_delete_thread): Replace accessing list
	implemention details with API function.
	(match_dll_by_basename): New function.
	(dll_is_loaded_by_basename): New function.
	(win32_ensure_ntdll_loaded): Replace accessing list implemention
	details call to dll_is_loaded_by_basename.