[PATCH, c++0x] Use explicitly defaulted ctor, cctor, dtor and op= for duration in <chrono>

[Chris Fairles]

Attached is patch that bring duration up-to-spec according to N2691
(current WD). This includes some additional diagnostics regarding
20.8.3 paragraphs 2, 3 and 4 as well as using explicitly-defaulted
definitions for durations ctor, cctor, dtor and op=.

These changes do not affect any of the other <chrono> changes required
for the whole clock_gettime/librt ordeal.

Chris
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