Picking time in the middle of the interval, not its edge, and reduce error bound.
Picking time in the middle of the interval, not its edge, and reduce
error bound.

- cntr/tools/cntr/cntr.c (measure): use the middle of a round-trip interval
  for the sample's nominal time
- cntr/tools/cntr/cntr.c (measure): error bound is only +/- RTT/2, not
  +/- RTT

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