Remove the "next" field from windows_thread_info

Remove the "next" field from windows_thread_info

This changes windows_thread_info to remove the "next" field, replacing
the linked list of threads with a vector.  This is a prerequisite to
sharing this structure with gdbserver, which manages threads
differently.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (struct windows_thread_info): Remove typedef.
	(thread_head): Remove.
	(thread_list): New global.
	(thread_rec, windows_add_thread, windows_init_thread_list)
	(windows_delete_thread, windows_continue): Update.