Complete use of unique_ptr with notif_event and stop_reply

Complete use of unique_ptr with notif_event and stop_reply

We already use unique_ptr with notif_event and stop_reply in some
places around the remote target, but not fully.  There are several
code paths that still use raw pointers.  This commit makes all of the
ownership of these objects tracked by unique pointers, making the
lifetime flow much more obvious, IMHO.

I notice that it fixes a leak -- in remote_notif_stop_ack, We weren't
destroying the stop_reply object if it was of TARGET_WAITKIND_IGNORE
kind.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Id81daf39653d8792c8795b2a145772176bfae77c