Do not pollute userspace with <sys/proc.h>, instead declare struct thread
when _KERNEL is defined.

Include <sys/time.h> instead of <sys/timespec.h>. This causes intentional
namespace pollution that mimics Linux. g/musl libcs include <time.h> in
their <sys/timerfd.h>, exposing clock gettime, settime functions and
CLOCK_ macro constants. Ports like Chromium expect this namespace
pollution and fail without it.