MINOR: init/threads: make the global threads an array of structs

MINOR: init/threads: make the global threads an array of structs

This way we'll be able to store more per-thread information than just
the pthread pointer. The storage became an array of struct instead of
an allocated array since it's very small (typically 512 bytes) and not
worth the hassle of dealing with memory allocation on this. The array
was also renamed thread_info to make its intended usage more explicit.