Improve clock_gettime and utimensat resolution.

Improve clock_gettime and utimensat resolution.

* hires.h (hires_ms): Change initime_us to initime_ns, with 10x
more resolution.
(hires_ms::nsecs): New prototype.
(hires_ms::usecs, hires_ms::msecs, hires_ms::uptime): Adjust.
* times.cc (systime_ns): New helper function.
(hires_ms::prime): Use it for more resolution.
(hires_ms::usecs): Change to...
(hires_ms::nsecs): ...with more resolution.
(clock_gettime): Use more resolution.
(systime): Rewrite in terms of systime_ns.
(timespec_to_filetime): Rewrite math to reflect true operation.
* fhandler_disk_file.cc (utimens_fs): Use higher resolution.