Raise ValueError when value being stored in a time_t variable will result in

Raise ValueError when value being stored in a time_t variable will result in
more than a second of precision.  Primarily affects ctime, localtime, and
gmtime.

Closes bug #919012 thanks to Tim Peters' code.

Tim suggests that the new funciton being introduced, _PyTime_DoubletoTimet(),
should be added to the internal C API and then used in datetime where
appropriate.  Not being done now for lack of time.