C++ify remote notification code

C++ify remote notification code

This C++ifies the remote notification code -- replacing function
pointers with virtual methods and using unique_ptr.  This allows for
the removal of some cleanups.

2019-03-06  Tom Tromey  <tom@tromey.com>

	* remote.c (struct stop_reply_deleter): Remove.
	(stop_reply_up): Update.
	(struct stop_reply): Derive from notif_event.  Don't typedef.
	<regcache>: Now a std::vector.
	(stop_reply_xfree): Remove.
	(stop_reply::~stop_reply): Rename from stop_reply_dtr.
	(remote_notif_stop_alloc_reply): Return a unique_ptr.  Use new.
	(remote_target::discard_pending_stop_replies): Use delete.
	(remote_target::remote_parse_stop_reply): Update.
	(remote_target::process_stop_reply): Update.
	* remote-notif.h (struct notif_event): Add virtual destructor.
	Remove "dtr" member.
	(struct notif_client) <alloc_event>: Return a unique_ptr.
	(notif_event_xfree): Don't declare.
	(notif_event_up): New typedef.
	* remote-notif.c (remote_notif_ack, remote_notif_parse): Update.
	(notif_event_xfree, do_notif_event_xfree): Remove.
	(remote_notif_state_xfree): Update.