remote: C++ify thread_item and threads_listing_context

remote: C++ify thread_item and threads_listing_context

This patch C++ifies the thread_item and threads_listing_context
structures in remote.c.  thread_item::{extra,name} are changed to
std::string.  As a result, there's a bit of awkwardness in
remote_update_thread_list, where we have to xstrdup those strings when
filling the private_thread_info structure.  This is removed in the
following patch, where private_thread_info is also C++ified and its
corresponding fields made std::string too.  The xstrdup then becomes an
std::move.

Other than that there's nothing really special, it's a usual day-to-day
VEC -> vector and char* -> std::string change.  It allows removing a
cleanup in remote_update_thread_list.

Note that an overload of hex2bin that returns a gdb::byte_vector is
added, with corresponding selftests.

gdb/ChangeLog:

	* remote.c (struct thread_item): Add constructor, disable copy
	construction and copy assignment, define default move
	construction and move assignment.
	<extra, name>: Change type to std::string.
	<core>: Initialize.
	<thread_handle>: Make non-pointer.
	(thread_item_t): Remove typedef.
	(DEF_VEC_O(thread_item_t)): Remove.
	(threads_listing_context) <contains_thread>: New method.
	<remove_thread>: New method.
	<items>: Change type to std::vector.
	(clear_threads_listing_context): Remove.
	(threads_listing_context_remove): Remove.
	(remote_newthread_step): Use thread_item constructor, adjust to
	change to std::vector.
	(start_thread): Use thread_item constructor, adjust to change to
	std::vector.
	(end_thread): Adjust to change to std::vector and std::string.
	(remote_get_threads_with_qthreadinfo): Use thread_item
	constructor, adjust to std::vector.
	(remote_update_thread_list): Adjust to change to std::vector and
	std::string, use threads_listing_context methods.
	(remove_child_of_pending_fork): Adjust.
	(remove_new_fork_children): Adjust.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add rsp-low-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add rsp-low-selftests.o.
	* unittests/rsp-low-selftests.c: New file.
	* common/rsp-low.h: Include common/byte-vector.h.
	(hex2bin): New overload.
	* common/rsp-low.c (hex2bin): New overload.