This implements `platform.UtimesFile` which is similar to futimes.
Before, we were using path-based functionality even though the call site
was for a file descriptor.
Note: right now, there's no obvious code in Go to invoke the `futimens`
syscall. This means non-windows isn't implemented at nanos granularity,
so ends up falling back to the path based option.
Finally, this removes tests for the seldom supported updates with
negative epoch time. There's little impact to this as setting times on
files before 1970 isn't a typical use case.
Signed-off-by: Adrian Cole <adrian@tetrate.io>
f5d194c43c