push remote_desc into struct remote_state

push remote_desc into struct remote_state

This moves the "remote_desc" global into remote_state.

	* remote.c (struct remote_state) <remote_desc>: New field.
	(remote_desc): Remove.
	(remote_threads_info, remote_threads_extra_info, remote_close)
	(send_interrupt_sequence, remote_start_remote, remote_open_1)
	(readchar, remote_xfer_partial, remote_rcmd, packet_command)
	(remote_hostio_send_command, remote_file_put, remote_file_get)
	(remote_file_delete, remote_can_async_p, remote_is_async_p)
	(remote_async, remote_new_objfile, set_range_stepping): Update.