unix,windows: fix timer order in case of same timeout

Compare start_id of timer handles when they have the same timeout.
start_id is allocated with loop->timer_counter in uv_timer_start.
unix, windows: make uv_fs_t.statbuf public

Make the statbuf field public. This means you no longer have to use
req->ptr - though that still works and will continue to work for the
foreseeable future.

Fixes #704.