Fix unlikely problems with time overflow

Fix unlikely problems with time overflow

Also, fix some rounding errors while we’re in the neighborhood.
* src/buffer.c (duration_ns, compute_duration_ns): Rename from
‘duration’ and ‘compute_duration’, and count ns rather than s, to
lessen rounding error.  All uses changed.
(compute_duration_ns): Work even if the clock moves backward
and time_t is unsigned.
(print_stats): Don’t worry about null or empty TEXT, as that
cannot happen.  Compare double to UINTMAX_MAX + 1.0, not
to UINTMAX_MAX, so that the comparison is exact.
Handle the unlikely case that numbytes >= UINTMAX_MAX.
* src/tar.c (parse_opt): Treat -L hugenumber as effectively
infinity rather than erroring out.
Prefer ckd_add to checking overflow by hand.