Move CLOCKS_PER_SEC to sys/_time.h so the kernel has access to it.

Move CLOCKS_PER_SEC to sys/_time.h so the kernel has access to it.
This will be used in waitid(2) to set si_utime and si_stime.
The definition of struct timespec also moves from time.h to sys/_time.h
for struct itimerspec.  OK kettenis@