Use std::list for event notifications in gdbserver

Use std::list for event notifications in gdbserver

This changes gdbserver to use std::list rather than common/queue.h for
event notifications.

gdb/gdbserver/ChangeLog
2019-04-19  Tom Tromey  <tom@tromey.com>

	* server.c (struct vstop_notif): Derive from notif_event.
	<base>: Remove.
	(queue_stop_reply): Update.
	(remove_all_on_match_ptid): Change type.  Rewrite.
	(discard_queued_stop_replies): Rewrite.
	(in_queued_stop_replies_ptid): Change type.
	(in_queued_stop_replies): Rewrite.
	(notif_stop): Update.
	(queue_stop_reply_callback): Update.
	(captured_main): Don't call initialize_notif.
	(push_stop_notification): Update.
	* notif.c (notif_write_event, handle_notif_ack)
	(notif_event_enque, notif_push): Update.
	(notif_event_xfree, initialize_notif): Remove.
	* notif.h (struct notif_event): Include <list>, not
	"common/queue.h".
	(struct notif_server) <queue>: Now a std::list.
	(notif_event_p): Remove typedef.
	(initialize_notif): Don't declare.
	(struct notif_event): Add virtual destructor.