Add thread_handle_to_thread_info support for remote targets

Add thread_handle_to_thread_info support for remote targets

This patch adds support to remote targets for converting a thread
handle to a thread_info struct pointer.

A thread handle is fetched via a "handle" attribute which has been
added to the qXfer:threads:read query packet.  An implementation is
provided in gdbserver for targets using the Linux kernel.

gdb/gdbserver/ChangeLog:

	* linux-low.h (struct lwp_info): Add new field, thread_handle.
	(thread_db_thread_handle): Declare.
	* linux-low.c (linux_target_ops): Initialize thread_handle.
	* server.c (handle_qxfer_threads_worker): Add support for
	"handle" attribute.
	* target.h (struct target_ops): Add new function pointer,
	thread_handle.
	(target_thread_handle): Define.
	* thread-db.c (find_one_thread, attach_thread): Set thread_handle
	field in lwp.
	(thread_db_thread_handle): New function.

gdb/ChangeLog:

	* remote.c (vector): Include.
	(struct private_thread_info): Add field, thread_handle.
	(free_private_thread_info): Deallocate storage associated with
	thread handle.
	(get_private_info_thread): Initialize `thread_handle' field.
	(struct thread_item): Add field, thread_handle.
	(clear_threads_listing_context): Deallocate storage associated
	with thread handle.
	(start_thread): Add support for "handle" attribute.
	(thread_attributes): Add "handle".
	(remote_get_threads_with_qthreadinfo): Initialize thread_handle
	field.
	(remote_update_thread_list): Update thread_handle.
	(remote_thread_handle_to_thread_info): New function.
	(init_remote_ops): Initialize to_thread_handle_to_thread_info.