It happens that people subtract times more than 292 years apart (usually because of use
of the zero time), resulting in an incorrect Duration. The current subtraction does not
check for overflow so the duration wraps and may be positive or negative but is
certainly not correct.

We can't return the correct answer, but we could help avoid surprises by clamping the
result of Sub to -maxDuration or +maxDuration.

This would apply to time.Since as well.

Too late for Go 1.1. Probably do for Go 1.2.