push general_thread and continue_thread into struct remote_state

push general_thread and continue_thread into struct remote_state

This moves the globals general_thread and continue_thread into
remote_state.

	* remote.c (struct remote_state) <general_thread, continue_thread>:
	New fields.
	(general_thread, continue_thread): Remove.
	(record_currthread, set_thread, set_general_process)
	(remote_open_1, extended_remote_attach_1, remote_wait_as)
	(extended_remote_mourn_1): Update.