Bug fix---compiler warnings.

On a 32-bit system, a [u]int64_t quantity can't be printed as a "long", but
as a "long long".  Also include a cast so we ensure we get the proper values
on a 32-bit stack.

Also fixed the definition of TWD_DEFAULT_DURATION to do all the calcuations
using long long.
Bug fix---format still wrong.

The quanties being printed out are defined as uint64_t, but the format string
was specified as a signed quantity.
Bug fix---again, a signed/unsigned problem.

Make sure we define TWD_DEFAULT_DURATION as an unsigned quantity.