use auto for nsecdur, assigning system_tp into steady_tp compiles but is
not correct. just change it to auto.
use {} initialization for timespec structs and returning std::pair.
in timediff, fromTimespec and toTimespec the else case was calculating
wrong. we need to correctly handle the borrow when the nanoseconds of
the first time are smaller than the second, by adding TIMESPEC_NSEC_PER_SEC
and decrementing the seconds.
