Use the more precise algorithm of GNU "make" to decide whether

Use the more precise algorithm of GNU "make" to decide whether
a file is in the future, by looking at high-resolution time
stamps if available.

(TIMESPEC_NS): New macro.
(current_time): Initialize to the minimum value.
(current_time_ns): New var.
(main): Do not bother to initialize current_time;
it's no longer needed.
(get_current_time): New function.
(print_long_format): Use it when a file appears to be in the future.
Get the nanoseconds of the file's time stamp, if available,
and use that to decide whether the file appears	to be in the future.