Report the seconds and microseconds values from the INSN_GTOD
instructions as unsigned values to postpone the Y2038 problem to a Y2106
problem. Be a bit more careful with the conversion to a signed
difference. We wouldn't normally expect time to go backwards, but it is
possible if the system time is set to to an older time between the
instructions.
Also, in the description of INSN_GTOD in the Comedilib manual,
explicitly mention that the seconds and microseconds values are
unsigned.
A struct timeval is not the same size as a pair of lsampl_t's for 64-bit,
so can't point 'data' in a comedi_insn to a struct timeval and expect it
to work!
