setitimer(2): don't round up it_value

setitimer(2): don't round up it_value

We can reduce latency for the first expiration of a timer if we don't
round it_value up to the minimum interval (1 tick).

While we're at it, we may as well consolidate all input validation and
adjustment into a single itimerfix() call.  There are no other callers
in the kernel (nor should there be), so remove the prototype from
sys/time.h.

Discussion: https://marc.info/?l=openbsd-tech&m=162084338005502&w=2

Tested by weerd@ and claudio@.

probably ok claudio@