This constifies the "buf" arguments to various remote-notif functions
and updates the users.

gdb/ChangeLog
2019-01-14  Tom Tromey  <tom@tromey.com>

	* remote-notif.c (handle_notification, remote_notif_ack)
	(remote_notif_parse): Make "buf" const.
	* remote-notif.h (struct notif_client) <parse, ack>: Make "buf"
	const.
	(remote_notif_parse, remote_notif_ack, handle_notification):
	Likewise.
	* remote.c (remote_notif_stop_parse): Make "buf" const.
	(remote_target::remote_parse_stop_reply): Make "buf" const.
	(remote_notif_stop_ack): Make "buf" const.