This fixes various date formatting problems when building on a system where time_t is 64-bit (e.g. Visual Studio 2010), and, as a bonus, extends the range past 2038 on such systems.
The wire protocol is left at 32-bit for now, we might change that to 64 when other reasons to break compatibility have accumulated.
The default value of the "dst" argument must be 0, because -1 leads to incorrect results when the given time falls into local DST. However, a "dst" argument makes no sense on a method that deals with GMT anyway, so remove it entirely.
