When converting ns,us,ms to sbt, return the ceil() of the result
rather than the floor(). Returning the floor means that
sbttoX(Xtosbt(y)) != y for almost all values of y. In practice, this
results in a difference of at most 1 in the lsb of the sbintime_t.
This difference is meaningless for all current users of these
functions, but is important for the newly introduced sysctl conversion
routines which implicitly rely on the transformation being idempotent.

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