Move pending_event to remote_notif_state.

Move pending_event to remote_notif_state.

This patch moves pending_event to remote_notif_state.  All pending
events are destroyed in remote_notif_state_xfree.  However,
discard_pending_stop_replies release pending event too, so the pending
event of stop notification is released twice, we need some refactor
here.  We add a new function discard_pending_stop_replies_in_queue
which only discard events in stop_reply_queue, and let
remote_notif_state_xfree release pending event for all notif_client.

After this change, discard_pending_stop_replies is only attached to
ifnerior_exit observer, so the INF can't be NULL any more.  The
NULL checking is removed too.

gdb:

2013-10-04  Yao Qi  <yao@codesourcery.com>

	* remote-notif.h (REMOTE_NOTIF_ID): New enum.
	(struct notif_client) <pending_event>: Moved
	to struct remote_notif_state.
	<id>: New field.
	(struct remote_notif_state) <pending_event>: New field.
	(notif_event_xfree): Declare.
	* remote-notif.c (handle_notification): Adjust.
	(notif_event_xfree): New function.
	(do_notif_event_xfree): Call notif_event_xfree.
	(remote_notif_state_xfree): Call notif_event_xfree to free
	each element in field pending_event.
	* remote.c (discard_pending_stop_replies): Remove declaration.
	(discard_pending_stop_replies_in_queue): Declare.
	(remote_close): Call discard_pending_stop_replies_in_queue
	instead of discard_pending_stop_replies.
	(remote_start_remote): Adjust.
	(stop_reply_xfree): Call notif_event_xfree.
	(notif_client_stop): Adjust initialization.
	(remote_notif_remove_all): Rename it to ...
	(remove_stop_reply_for_inferior): ... this.  Update comments.
	Don't check INF is NULL.
	(discard_pending_stop_replies): Return early if notif_state is
	NULL.  Adjust.  Don't check INF is NULL.
	(remote_notif_get_pending_events): Adjust.
 	(discard_pending_stop_replies_in_queue): New function.
	(remote_wait_ns): Likewise.