gdbserver: use std::list for all_threads

Remove the usage of inferior_list for the all_threads list in
gdbserver.  The entry field in thread_info is removed, and replaced by a
simple ptid field.

I added some functions to iterate (for_each_thread) and find threads
(find_thread).  However, changing all the users of find_inferior & co to
use these new functions would have made the patch way too big.  So I
opted instead to make find_inferior & co some shims, so that the
existing code only needs to be updated minimally.  We can then update
the existing code to use the new functions incrementally (I've started
to do the work, but I'll post it afterwards, see [1] if you want a peek).

This patch has been built-tested on all relevant platforms, except
lynx.  I also regtested using the native-gdbserver and
native-extended-gdbserver boards on x86.

[1] https://github.com/simark/binutils-gdb/commits/kill-inferior-list-entry

gdb/gdbserver/ChangeLog:

	* inferiors.h: (struct inferior_list): Remove.
	(struct inferior_list_entry); Remove.
	(add_inferior_to_list, clear_inferior_list, one_inferior_p,
	A_I_NEXT, ALL_INFERIORS_TYPE, ALL_INFERIORS, remove_inferior,
	get_first_inferior): Remove.
	(for_each_inferior, for_each_inferior_with_data, find_inferior,
	find_inferior_id, find_inferior_in_random): Change signature.
	* inferiors.c (all_threads): Change type to
	std::list<thread_info *>.
	(get_thread): Remove macro.
	(find_inferior, find_inferior_id): Change signature, implement
	using find_thread.
	(find_inferior_in_random): Change signature, implement using
	find_thread_in_random.
	(for_each_inferior, for_each_inferior_with_data): Change
	signature, implement using for_each_thread.
	(add_inferior_to_list, remove_inferior): Remove.
	(add_thread, get_first_thread, thread_of_pid,
	find_any_thread_of_pid, free_one_thread, remove_thread): Update.
	(get_first_inferior, one_inferior_p, clear_inferior_list):
	Remove.
	(clear_inferiors, get_thread_process): Update.
	* gdbthread.h: Include <list>.
	(struct thread_info) <entry>: Remove field.
	<id>: New field.
	(all_threads): Change type to std::list<thread_info *>.
	(get_first_inferior): Add doc.
	(find_thread, for_each_thread, find_thread_in_random): New
	functions.
	(current_ptid, pid_of, ptid_of, lwpid_of): Update.
	* linux-arm-low.c (update_registers_callback): Update.
	* linux-low.c (second_thread_of_pid_p): Update.
	(kill_one_lwp_callback, linux_detach_lwp_callback,
	delete_lwp_callback, status_pending_p_callback, same_lwp,
	find_lwp_pid, num_lwps, iterate_over_lwps_filter,
	iterate_over_lwps, not_stopped_callback,
	resume_stopped_resumed_lwps, count_events_callback,
	select_singlestep_lwp_callback, select_event_lwp_callback,
	unsuspend_one_lwp, linux_wait_1, send_sigstop_callback,
	suspend_and_send_sigstop_callback, wait_for_sigstop,
	stuck_in_jump_pad_callback, move_out_of_jump_pad_callback,
	lwp_running, linux_set_resume_request, resume_status_pending_p,
	need_step_over_p, start_step_over, linux_resume_one_thread,
	proceed_one_lwp, unsuspend_and_proceed_one_lwp,
	reset_lwp_ptrace_options_callback): Update.
	* linux-mips-low.c (update_watch_registers_callback): Update.
	* regcache.c (regcache_invalidate_one, regcache_invalidate):
	Update.
	(free_register_cache_thread_one): Remove.
	(regcache_release): Update.
	* server.c (handle_btrace_enable_bts, handle_btrace_enable_pt,
	handle_qxfer_threads_worker): Update.
	(handle_query): Update, use list iterator.
	(visit_actioned_threads, handle_pending_status,
	queue_stop_reply_callback, gdb_wants_all_threads_stopped,
	clear_pending_status_callback, set_pending_status_callback,
	find_status_pending_thread_callback, handle_status,
	process_serial_event): Update.
	* target.c (thread_search_callback): Update.
	* thread-db.c (thread_db_get_tls_address): Update.
	* tracepoint.c (tracepoint_finished_step, tracepoint_was_hit):
	Update.
	* win32-i386-low.c (update_debug_registers_callback): Update.
	* win32-low.c (delete_thread_info, child_delete_thread,
	continue_one_thread, suspend_one_thread,
	get_child_debug_event): Adjust.